Building a 100 album vinyl collection 3 must have albums are?

The Doors First ,Rolling Stones  Let it Bleed ,Santana First ...

lately, it's 

A Day at the Races

Let it Bleed

His Band and the Street Choir

There are absolutely no "must-have" albums. None, zero, zilch.

Find three albums that YOU like, that is all that matters. If you don't know what you like, wait, buy nothing. Find out what YOU like and then get those.

I like to support new artists on Bandcamp, so possibly head over there and take the plunge. Most of it is not my taste, so it takes a bit of time to try a bunch of different artists/bands/genres. Occasionally, there may be some unexpected happy surprises. Good luck with your journey of music discovery!