3-hour flight with about 75% or so IMC?
I haven't had my IMC rating for that long but 2.25hrs in cloud I think would be very tiring...my experience is that if you get into very bumpy IMC then almost all your concentration goes on controlling the plane with little extra metal capacity for anything else.
I have done more than that under the hood but that was with my instructor doing the nav and radio. In real life I'd say 2.25hrs is too much.