Changing "and" to "&" and removing Oxford comma in bibliography
If it makes sense I'd suggest using a language that does not have the Oxford comma (british for example).
But if that is not an option, a simple \let\finalandcomma=\empty in \DefineBibliographyExtras{english} should do it:
\DefineBibliographyExtras{english}{\let\finalandcomma=\empty}
If we use that, we do not even need \renewcommand*{\finalnamedelim}{\addspace\bibstring{and}\space}.
I have also changed the redefinition of \finalnamedelim to \DeclareDelimFormat[bib,biblist]{finalnamedelim}{\addspace\&\space}, as the test added nothing.
\documentclass[12pt]{article}
\usepackage[
backend=biber,
citestyle=numeric,
isbn=false,
bibstyle=authoryear,
maxnames=3,
minnames=3,
giveninits=true,
terseinits=true,
sorting=none]
{biblatex}
\DeclareFieldFormat[article]{volume}{\mkbibbold{#1}\addcomma\space}
\DeclareFieldFormat{pages}{#1}
\renewcommand*{\revsdnamepunct}{}
\DeclareDelimFomat[bib]{nametitledelim}{\addspace}
\DeclareFieldFormat[article]{title}{#1}
\DeclareNameAlias{sortname}{family-given}
\DeclareDelimFormat[bib,biblist]{finalnamedelim}{\addspace\&\space}%
\DefineBibliographyExtras{english}{\let\finalandcomma=\empty}
\usepackage{xpatch}
\xpatchbibmacro{name:andothers}
{\bibstring{andothers}}
{\bibstring[\emph]{andothers}}
{}{}
\renewbibmacro{in:}{%
\ifentrytype{article}
{}
{\printtext{\bibstring{in}\intitlepunct}}}
\defbibenvironment{bibliography}
{\list
{\printtext[labelnumberwidth]{%
\printfield{labelprefix}%
\printfield{labelnumber}}}
{\setlength{\labelwidth}{\labelnumberwidth}%
\setlength{\leftmargin}{0pt}%
\setlength{\labelsep}{\biblabelsep}%
\setlength{\itemsep}{\bibitemsep}%
\setlength{\itemindent}{\labelwidth}%
\addtolength{\itemindent}{\labelsep}%
\setlength{\parsep}{\bibparsep}}%
\renewcommand*{\makelabel}[1]{\hss##1}}
{\endlist}
{\item}
\DeclareFieldFormat{labelnumberwidth}{#1\addperiod} % Adds a period after each reference number in bibliography
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{author2014,
author = {Author, A. B. and Author, A. B. and Author, A. B.},
journal = {Journal of the happy but frustrated biblatex user},
pages = {1070--1074},
title = {{Biblatex hurts so good}},
volume = {10},
date = {2014}
}
@article{author2013,
author = {Author, A. D. and Author, A. B. and Author, A. B. and Author, A.B},
journal = {Journal of the happy but frustrated biblatex user},
pages = {1070--1074},
title = {{Biblatex hurts so good}},
volume = {10},
date = {2013}
}
\end{filecontents*}
\addbibresource{\jobname.bib}
\begin{document}
Be excellent, Dudes \supercite{author2013,author2014}.
\printbibliography
\end{document}
You have solved much of the original problem yourself; you can solve the last bit to remove the Oxford comma before the et. al. in the bibliography entries with the following code added to your MWE:
\xpatchbibmacro{name:andothers}{ % Removes comma before the "et al"
{\finalandcomma}%
}{%
\addspace%
}{}{}
This gives:
The final working example becomes:
\documentclass [12pt]{article}
\usepackage[
backend=bibtex,
citestyle=numeric,
isbn=false,
bibstyle=authoryear,
maxnames=3, % Max number of names before "et al" starts
minnames=3, % Must show at least three names for citations and bibliography
firstinits=true, % Removes period after first name intial
terseinits=true, % Removes period after first name intial
sorting=none] % List citations in order they appear
{biblatex}
\DeclareFieldFormat[article]{volume}{\textbf{#1}\addcomma\space} % Bolds volume of citation and adds comma after volume
\DeclareFieldFormat{pages}{#1} % No prefix for the "pages" field in the bibliography
\renewcommand*{\revsdnamepunct}{} % Removing the comma after last name in bibliography
\renewcommand*{\revsdnamepunct}{} % Removing the periods after first name intial in bibliography
\renewcommand{\labelnamepunct}{\addspace} % Removes period after year in bibliography
\DeclareFieldFormat[article]{title}{#1} % Remove quotes from journal title in bibliography
\DeclareNameAlias{sortname}{last-first} % Lists all authors with last name first in bibliography
\renewcommand*{\finalnamedelim}{\addspace\bibstring{and}\space} % Removes comma after penultimate name
\AtBeginBibliography{% Replaces "and" with "&" in bibliography
\renewcommand*{\finalnamedelim}{%
\ifnumgreater{\value{liststop}}{2}{}{}%
\addspace\&\space}%
}
\usepackage{xpatch}
%<<<<<<<<<<<<<<<<<<<<<<<<<<
\xpatchbibmacro{name:andothers}{ % Removes comma before the "et al"
{\finalandcomma}%
}{%
\addspace%
}{}{}
\xpatchbibmacro{name:andothers}{ % Makes the "et al" italicized bibliography
\bibstring{andothers}%
}{%
\bibstring[\emph]{andothers}%
}{}{}
\renewbibmacro{in:}{% Removes "In" from bibliography entries
\ifentrytype{article}{}{%
\printtext{\bibstring{in}\intitlepunct}}}
\defbibenvironment{bibliography}
{\list
{\printfield[labelnumberwidth]{labelnumber}} % Adds numbers to references and wraps the text for each bibliography entry
{\setlength{\labelwidth}{\labelnumberwidth}%
\setlength{\leftmargin}{0pt}%
\setlength{\labelsep}{\biblabelsep}%
\setlength{\itemsep}{\bibitemsep}%
\setlength{\itemindent}{\labelwidth}%
\addtolength{\itemindent}{\labelsep}%
\setlength{\parsep}{\bibparsep}}%
\renewcommand*{\makelabel}[1]{\hss##1}}
{\endlist}
{\item}
\DeclareFieldFormat{labelnumberwidth}{#1.} % Adds a period after each reference number in bibliography
\usepackage{filecontents}
\begin{filecontents*}{test.bib}
@article{author2014,
author = {Author, A. B. and Author, A. B. and Author, A. B.},
journal = {Journal of the happy but frustrated biblatex user},
pages = {1070--1074},
title = {{Biblatex hurts so good}},
volume = {10},
date = {2014}
}
@article{author2013,
author = {Author, A. D. and Author, A. B. and Author, A. B. and Author, A.B},
journal = {Journal of the happy but frustrated biblatex user},
pages = {1070--1074},
title = {{Biblatex hurts so good}},
volume = {10},
date = {2013}
}
\end{filecontents*}
\bibliography{test.bib}
\begin{document}
Be excellent, Dudes \supercite{author2013,author2014}
\printbibliography
\end{document}