Ditto on the iPad 3 experience, I was so sorry for giving up on 6.1.3 and then 7.1.2. I deleted the IPSW files and do knock myself for doing that.
I'm going to perform a clean install on each device I own before installing 8.3, however. I have a BT keyboard I can use so entering in my credentials should go a little more smoothly...
Two things I'll give you some hope on for the iPad 3 and iOS 8.3, I'm using several Adobe CC apps (with the Paper and Wacom Creative 2 styli) on both iPads and I can't really tell the difference between my Air and the iPad 3. And, I actually turned off the "Accessibility>Reduce Motion" option on my iPad 3 - so a lot of the effects are enabled, and the iPad 3 is still very usable. I'm happy with the latest iteration of iOS 8, and I'll be holding off on iOS 9 - I've learned that lesson! Cheers!