I see a real disconnect between the first part of your paragraph and the second part. On one hand, you say (and rightfully so) that your son is wonderful, while acknowledging his disability (being nonverbal). You state that you learned patience and how to be positive about everything.
Then you go on to describe your new foster children, with whose care you've been entrusted for less than 3 months. And you've described them as having a horrible attitude, and being complete whiners, crying, never doing what they're told, and never happy.
A few weeks, or even one week, is a terribly short time, and although you don't provide info about what kind of life these children led prior to being placed in your home, I'm guessing it couldn't have been happy, stable, or loving (since they're now in foster care). Even if they weren't abused or neglected, maybe they experienced the traumatic loss of a parent.
Please apply the same patience and positive thinking that you express to your son to the three new foster children. Perhaps the whining is how they got any kind of attention in their birth parents' home. Maybe they're crying because their little precious lives have been uprooted and they now live with strangers and everything's different. A 3 year old under the best circumstances doesn't do what he's told - he's too busy finding out that he can run and hide and make mommy a little nuts, and giggle in the most inappropriate places and times, and be mischievous, and pull the kitten's tail, and all those crazy, fun, and yes - tiring - things that 3 year olds do. Now you combine a 3 year old brain with a complete upset in parent figures, houses, rooms, food, schedule, siblings, words...who wouldn't cry and whine and act out?!!??!
Let up on the time-outs, and just show them stability, security, boundaries, consistency, and love. I'm assuming you have access to social workers and counselors and foster care training.