If mom works then i would think its okay but if she is staying at home then IMO its something that can wait till 4 - or just maybe 6 months before pre-k, they will pick up on the social aspects and such quickly..no sense spending the money if its not needed.
Our first two the wife worked very little and we put them in day care around 4. Seems getting them used to being dropped off and left for a few hours is more important than the "skills" they claim to teach them.
For one year we had all three in day care when the wife went back to work full time, that totally sucked! We ate a lot of roman...bur got better as they went into the government schools my taxes were already paying for.
Our 3rd was in day care since he was 6 months, always felt bad for him, but he is the most adjusted one we have, he'll probably be in sales one day, has no fear to go up to anyone and talk to them where my middle child doesn't even like to order food from a person (shes 15)