The issue is in how Bootstrap 3 sets up those boxes (technically, the
col- divs... the
thumbnail divs are along for the ride on this one). BS3 uses floats to achieve its grid system, which are near-impossible to get "bottom-aligned".
Two options come to mind. One is to switch to Bootstrap 4, which uses flexbox for much of its grid system. But it will mean a re-write of much of your class calls. Some of those names have changed and some were dropped.
Option 2 is to learn flexbox and write some special styles just for that section of your page. Doing this means you have to re-write both
md-4 classes along with the media queries that make that work, but it's far easier than starting from scratch.
Best place to learn flexbox right now? https://css-tricks.com/snippets/css/a-guide-to-flexbox/
A basic untested example:
flex-flow: row wrap;
align-items: stretch; /* should be default, but just in case */