Is it OK to put hardwood flooring over 3/4" particle board that's over 1/2" wood subfloor?
1/2" subfloor is definitely insufficient, so I get why it was reinforced.

3/4" particle board... that is not structural.  It's paper dust and glue.  A little water and it loses all cohesion.

Simply for peace of mind, if it were me, I'd remove the particle board and put down either OSB or plywood.  Probably OSB, only because it's a little stronger (not much) but it's also more expensive so if budget were a concern, I'd go with equal thickness plywood.  Either will do you.

But the particle board?  Get rid!  Its probably not gonna fall apart on you, unless you have a spill... but why even keep that worry?