C.H.
Our preschool would be willing to take kids at 2 years, 10 months old as long as they are potty trained. The odd thing is that they would be in with the 3s and then then next level up would be 4s. They will not be able to start "real school" until they are 5, so if you choose to enroll them earlier, they may very well have to repeat either the 3s or the 4 year old class (unless of course you keep them out of school the last year).
Does that make sense? It happened with us. My son turned 3 in January, so he attended the last 1/2 of the 3s and then the next year, we had a choice for him to stay in 3s or move him to 4s. We opted to keep him in 3s since he was only technically 3.5.