Polyglot the (non constant) OEIS!
3 languages (1 byte) - Pyth (A001477), MATL (A000027), Braingolf (A000217)
Q


Try Pyth online!, 0-indexed (non-negative integers)
Try MATL online!, 0-indexes (positive integers)
Try Braingolf online!, 1-indexed (triangular numbers)


How?
Q does the following:

In MATL, it means + 1, so it basically evaluates to input + 1.

In Pyth, it means input, so it just outputs the input.

In Braingolf, it is the built-in for triangular numbers.


Mini-polyglot
Pyth can be replaced by any of the following languages: GolfScript, Pyke, Brachylog or Brain-Flak.