Are all 3-4-5 (Pythagorean triples) triangles right triangles? Or if we are told it is a 3-4-5 triangle, can we not assume right triangle. Thanks!

yes ... all 3-4-5 triangles are right triangles.

yes.

it works the other way around, too:
if you have a right triangle with legs in a 3:4 ratio, then it has to be 3:4:5 overall. same goes for a right triangle with leg:hypotenuse = 3:5 or 4:5.

beware of 'impostor' triangles, though. if you see a right triangle in which the ratio of leg to hypotenuse is 3:4, then it's not a 3-4-5 triangle, because the 'five' part of the ratio has to be on the hypotenuse.