Are 3 2x6's as strong as a 6x6 post And if using 2x6s is it strong enough for a beam to span 12 feet Posts and beams of 2x6s to b staggered to create a tongue and groove type connection for pole barn?

Laminating 2x6's is a much better idea, it won't twist like a PT 6x6, it is less expensive, and you can use a combination of lumber. I built a 24x 40 ' barn a few years ago for a woodworking shop. I used 4 pieces of 2x6, this gave me a doubled space at the top to slide in laminated doubled 2x12's ... I used 0.40 PT lumber at the bottom and regular KD above, I laminated the bottom with staggered board lengths... I used a 12, 10 and two 8 footers, I needed an 18 ' post to have 4' in ground and 14' above. I also used stainless steel nails below ground and regular above, a nail gun sure was handy. After the posts were in, I drilled holes and used a 6 inch 3/8th carriage bolt to reinforce the connection. I overbuilt my barn, went 8' on center, not 12'. Also I used a doubled 2x14 glue lam 40' long for the ridge, that way the building was clear span and I didn't put up collar ties. I used a regular plywood roof and shingles. I used one inch rough cut shiplap pine boards for vertical sheathing, came out nice.