AND, OR gates: 3 transistors. NAND, NOR gates: 2 transistors. Why?
The premise of your question is false. Here's an OR gate with zero transistors and two diodes:







simulate this circuit &ndash; Schematic created using CircuitLab

This isn't an especially good OR gate, but it is one. It doesn't have any voltage gain, for starters.

It can be the case that some logic designs require three transistors for an OR gate. This isn't anything intrinsic to an OR gate: it's just how that particular logic was designed. For whatever design goals the logic had, three transistors was the best way to do it.