LÖVE

I use love.graphics.polygon to draw triangles. But how can I check if touch press is in a triangle?

Simple. You need to detect a collision between a triangle shape and a point. Löve doesn't have a built-in function for that. The internet has tons of examples though, just search it and adapt the code to lua.

Point in triangle is a relatively simple check:
https://2dengine.com/?p=intersections#P ... e_triangle
If you want the touch area to be larger (circle) then it's slightly more complicated:
https://2dengine.com/?p=intersections#C ... s_triangle