Hello, you'll want to verify this with Tommudd but...
If you have a 2.5 inch kit, you could get the other 0.5 inches with top plates and/or clevis lift. If you lift 0.25 inches at the spring/clevis, it will buy you 0.5 inches of lift at the front wheel because of the suspension geometry.
In the rear you could use a leveling kit or stack 2-3 spring isolators to get the extra half inch.
Again, verify with Tom or Bmxer or one of the other Jeepers who has done OME lifts.

As for tires, stick with 245/75/R16. I am running those with my 4 inch kit. Any larger and you'll need to regear, and at 3 inches of lift 265's will rub. You would need 4 inches to run 265/75/R16's without rubbing, and again, you'd need to regear or suffer significant power loss and very low MPG.

How's the snow up in Vermont these days? It has been a while, but it is always fun driving US-4 over Killington gap.

I would do a 1/4 inch top plate ( for the extra .500 inches of lift)
With that done with the lift installation, you can always do a small clevis if you need to pretty easy
rear add one to start upper iso per side , should get you right in there

They beat me to it. If you plan to eventually upgrade to al's a-arms and shoot for 3.5" of lift, then get the 1/4" spacer plate and install it with your lift. In the rear add an extra iso per side. That'll get you 3" and a level stance. Just make sure you order 927 and 731 springs.

If the kit was installed already and you only plan on 3", then run out to Lowes or Home Deport and get a few 2" conduit nuts. Add 2 or 3 per side for either a 1/2" or 3/8" clevis lift that will get you up to 3/slightly over 3".

As Lauren said, stick with 245/75-16 or 265/70-16. Both are about the same height, the 265 is just a bit wider. I honestly would have went 265/70-16 when I first got tires, but they didn't sell Duratracs in that size at the time. You won't have rub issues, and there won't be a need to regear. You always could go to 4.10s if you ever want though.