3 oz at flameout is not terribly high. a little but shouldn't cause bitterness as not much of the alpha acids will isomerize when added then. how much did you add at 60 minutes? the more likely culprit of too bitter.
If memory serves me, (I'm at work so i don't have the recipe sheet or access to beersmith), I believe bittering was 1oz Chinook at 60min. Don't remember the exact alpha acid content.
The recipe was a Celebration Ale clone. I mashed at 157 for 60min (C60 and 2-Row), maybe 1.5 - 1.6 qt/gal. Batch sparged with 2 steps 0.61 gal and 1.5 gal (I'm pulling those numbers from my memory bank at the moment). Added 2lb of DME before boil, and added another 2.5 with 30 min left? Maybe I didn't stir the extract in well enough and some of it burned on the bottom of the pot? (I do remove the pot from the heat). Anticipated OG was 1.066 and I got 1.063.
Fermentation was probably 10 days in primary, 10 days in secondary with dry hops the last 7 days (Centennial/Cascade). Temp for primary was between 65-70. (Swamp cooler so it did flucctuate) I let it go to 70-71 at the end of the primary.