Assumed situation
Blinds 200/400 ante 50 Hero stack 15 000 M=15

Hero raises 3bb and a smaller stack moves all in for total 6bb
What do you need to call
Assumption 1 the smaller stack moved with top10% hand
The following are favourits to win and hence must call TT+,AQ+ thats only 9 hands AA,KK,QQ,JJ,TT,AKs,AQs,AKo,AQo. In all other situations pot odds must decide.
Assumption 2 the odds. Pot = 9x50+200+400+1200+2400=4650
you must lay 1200 more in to a 4650 pot odds 3.8:1
Assumption 3 if expectation is positive then (P is win %)
Px4650>1200. So P>26%
So to have positive expectation you should call with all hands that have more than 26% chance against the 10% range of the villain.
These cards are an incredibly large number including Ax,Kx,Qx,J3+
It definately includes all of the cards i would have made the raise with in the first place.

So Lesson 2. If you get re raised behind by a small stack (all in) you should probably always call