Sounds very unreasonable to expect a child to sit that long maybe suggest your church has a nursery arranged or you get a baby sitter.. plenty of time when she is older to go with you.
We also attended an Easter mass because of pressure from family, even though I thought it was absolutely crazy to even try to sit through a 3 1/2 hr mass with my 2 1/2 yo son lol We ended up going home half way through, after constantly taking him out of mass(i had reached my limit of that lol.) i think we were lucky we had even made it that far since my son is very very active lol
I think an at most 5 min attention span would be a reasonable expectation with boys especially. My son has about a 2-3 min attention span, and I talked to the doc recently and she said that was completely normal for a boy his age, and not to expect much beyond that.
It's definitely unreasonable to think a 2 1/2 yo can sit still for 3 hours without complaint(as i'm sure you know lol)