As the Donger, one does not simply get ganked 3v1 and not make a decent trade. If you go down, and still kill at least 2 of their guys (or at least one), you have done a good thing (it takes pressure off of one of the lanes, or all if it was the jungler).
I play Donger a lot, I stay in my mid lane and push unless a teammate asks for me to help, or I notice a lane is struggling and our jungler is twiddling his thumps in a bush somewhere. I max turrets/rockets first (depending on who I am laning against), and just stay in my little base while placing a turret up further ever chance I get.
How I set up my turrets is: First close to one side of river, the next other side, and the third ahead in the middle (if top, two nearest river, one in the middle near the bushes). This protects you from ganks, since (unless they stop to take out your turrets) they will be taking damage the entire time they are coming at you. If they stop to take your turrets, E then W, and replace your turret close to you, then run around like a lunatic until your turrets take them out. You are easiest to gank when you are mid, so you have to prepare your Donger for it.
R+Q can catch some gankers off guard, especially if they don't stop to focus your turrets. R+W is great against the person you are laning with when he comes into range. R+E... Eh, I rarely use it, but it is great for slowing down enemies that are out of range and trying to escape your turrets.
I like to always make sure that all three of my turrets have an intercepting point, where if the enemy steps in they will be hit by all 3 for maximum damage. If they step into it, E then W and you are good to go.
As for split pushing, as Donger I suggest it. If your team pushes your lane, either choose to stay and help push or go top and split push. Donger is great at pushing. If your team thinks you are "bad" for split pushing, then they are idiots. I usually feel that it is up to the top laner to split push, but that is more just my personal opinion than anything.
When split pushing, you should not worry about ganks as much, since the point of split pushing is to get the enemy team to focus the main push unite, while you take turrets in another lane. While I say you should not worry about ganks, you may still get ganked. But it would probably be easier to take them out since if they focus on you, they could lose an inhib. So at most you'd go 2v1, which with Donger is very easy to handle (especially melee champs). If they do focus you, though, your job then should be to keep them busy (whether you die or not), and give your team time to rush the lane.
Dunno if this helps, hell you probably know all this, but its still worth a try to help out a fellow player. :)
you naturally push lanes due to your turrets. On every back get at least one ward, and don't forget to upgrade to 
at lvl 9. Also, when a 3 man gank happens let your team know that there are 3 top so they can get a tower mid/bot or drag.