Choosing between MS Word, PDF, and latex formats when submitting a manuscript for publication when all are allowed?

That all depends on the journal you're submitting to. Quite some journals that accept Latex, also provide a template to use. With a template available and knowledge of Latex, it should be fairly easy to convert the GoogleDoc to Latex if you download as HTML and then use eg Pandoc to convert to Latex (explained here).

But people who are not comfortable with Latex, are likely better of using another option. Converting from GoogleDocs to .doc or .docx generally also requires an in-between step using PDF, and will generally give some layout issues, but when a paper is bound to be submitted it's not really the time to learn Latex.

TL;DR: Two considerations may speak for Word: possible differences between submission and publication requirements, and ease of collaboration and review.

The answers so far have sung the praises of LaTeX, and with good reason, but here is a caveat. You should try and determine whether the publication has a different requirement for the final version than the submission, just in case.

This happened to me with IEEE Computer last year. I have an article appearing in the July issue. When it was submitted about a year ago, I used LaTeX (despite some of my coauthors being unfamiliar with it), but when the magazine accepted it, they required me to convert it to Word! I found a tool to help with this but it was still a lot of extra effort.

I imagine this requirement arose because the magazine uses a professional editor to revise it prior to publication. Most publications are in journals rather than magazines and therefore aren't faced with this. (Or perhaps some journals do this too.). But because your question hit close to home and there will be some for whom this is applicable, I'm mentioning here.

Putting aside the question of whether a Word might actually be required, another point is the ease of collaboration and review. In this case, the OP has chosen to use Google Docs to edit, which certainly makes tracking changes and comments easy. Sending around Word has that effect too, whereas LaTeX requires a lot of hacks (macros to include or omit comments, latexdiff to markup changes, etc.) it's not like you're done as soon as you submit: you may quite likely have revisions to deal with.

Bottom line: while I find it easier to prepare initially in LaTeX, if you need to move from Docs to either Word or LaTeX, it seems likely it's easier to switch to Word and continue the submission cycle from there.

It depends a bit on the journal, but after acceptance most journals do the entire formatting from scratch. They may give you LaTeX templates, but that's just to give you an idea of what it will look like in the end and how many pages your paper will consist of.

There may be a big benefit to producing it in LaTeX though, but this depends a bit on the field you're in. In most STEM fields it is considered more professional than Word, so if you're considering sharing it as a preprint with colleagues this might be a factor that you want to weigh.