Our daughter got her glasses around her first birthday. The biggest issue at first is the glasses are uncomfortable and they will keep taking them off. At one year old, all we could do was keep putting them back on over and over and over. At three, you probably have more options, like the same incentives you used for potty training.
At any rate, the uncomfortable stage only lasted a week or two, then if the glasses came off it was by accident, maybe once per day for our daughter. We have friends with a son who got glasses at a very early age, and they had to get a strap to keep his from falling off, since he is so active.
The other thing to keep in mind is just like everything else, your son will outgrow his glasses pretty quickly. Adults can generally wait for their prescription to change, or the glasses to wear out. My eye insurance will pay for a new pair every two years for me, but your son may need his replaced more frequently than that. If he suddenly stops wanting to wear them after doing good for a while, they have probably become too tight.