All suggestions and commisserating welcome

DS is three. Most of the time he is an easy going guy but lately he's been having meltdowns, especially with transitions (usually when we have to go somewhere). Sometimes I can diffuse the situation early if I get the hint there is going to be a problem (he has a little scowl that comes out first). I've been trying to catch him on good days and reward him. For example, he likes to play on my kindle but his time with it is limited. He has had some meltdowns regarding getting out of the car when we get home from school (I don't get why he wants to stay in the car). We had a great day where he listened and came inside without any problem so I told him what a great job he did and let him play with the kindle while I made dinner. A few days later, he asked to play with the kindle on the way home. I told him he could if he listened and made good choices. He started to give me a problem getting out of the car, I reminded him to make good choices and he got out of the car no problem. I rewarded with the kindle again. He responds to this, if it is immediate.
Other things I've done:
Give him a countdown: "in 5 minutes we have to leave. In 3 minutes...in 2 minutes etc" Variable success with this.
Give him choices: "do you want the car pajamas or dinosaur pajamas?" Again, variable results.
Consequences: does not work at all. I don't think he can understand this concept right now. (Unless it is something immediate).
What do you do when your child is in the middle of a meltdown? Generally I put him in a safe place and ignore him but sometimes that isn't possible (for example if he doesn't want to leave to go to school, he doesn't have a choice. I have to go to work). The kindle is a powerful motivator right now but I haven't said "do xyz ans you can have kindle time." Should I? Today, he melted down when we had to leave to go to my husband's grandmother's for dinner. It went on forever.
Thanks in advance! I hope my post made sense (I had some much needed wine after bedtime tonight)
Re: meltdowns
This is just an idea, but for the car: DS will flip out if I turn off the engine and he was listening to the song playing. Not every time, but often enough that it's predictable. Could that be a reason he doesn't want to get out of the car?
@KatieB19, thank you! I'm not sure what the issue is with leaving the car. I'm going to have to pay more attention to see if there is a pattern. We usually chit chat the whole ride home. Maybe he just doesn't want it to end.
I also do say: "two more trips down the slide..." And that seems to work. I've been trying the 5 min, 3 min, 2 min, even though I know he has no idea what that means because it gives him multiple warnings. I'll have to try some different wording and see what works best.
You nailed it with the choices. Sometimes he won't make the choice and I have to make it for him.
@TiffanyBerry: DS doesn't like to get dressed or eat breakfast right away either
@jkep5909: I don't think it's fair when they start the terrible twos before they are two! Lol!
I have to remember that when he is in the midst of the epic meltdown, he will be impossible to reason with