i was wondering if putting 3 inch blocks in the rear will hurt the vehicle in any way, also will it affect my towing ability at all

 

What kind of vehicle do you have? What is the wheel-base? What type of drive-shaft? Does it already have a block? How tall is the block? Do you want to remove your current block and replace it with the new block? Are you wanting to stack the blocks on top of one another?

 

i have a 2000 f150 7700 4x4 ext cab with the 6.5 foot bed with everything is stock right now

 

Assuming you're wanting to replace your 2 inch block with a 3, you'll be fine. If you're wanting to stack them.....then you may run into a few issues. I don't think there are any aftermarket 3" blocks for your truck that have a cast in bump-stop like the factory blocks. A good solution is to use a set of 3" Ranger blocks. They are very heavy-duty cast iron and look just like your factory blocks except they are an inch taller.