How can I force ToC not to end with a chapter?

insert before the third chapter:

\addtocontents{toc}{\protect\newpage}

Another solution is to introduce stretchability to the space above chapters in the TOC, something like

\setlength{\cftbeforechapterskip}{1.0em plus 0.3em minus 0.1em}

That is what I normally do.

I found this question because I had the same issue. The response by @daleif didn't fix it for me, but I found this to work instead:

\renewcommand{\cftchapterbreak}{\addpenalty{-4000}}

The number you choose in place of 4000 determines how likely the new page will be before a chapter in the ToC. Out of curiosity, I set it to 10000 and all of my ToC chapters disappeared. High values (but under 10000) will prevent any chapter from crossing the page in the ToC. The might be problematic if you have numerous entries below a chapter that require a page break - I have not tested that yet.