The D3 is just too small for this type of work. The problem is how heavy the branches get and the resistance to the blade when pushing them. You will only be able to go so far before you become overloaded. If that's all that you have to use, then I would try to make lots and lots of small piles over the worse of the stumps.
The stumps will make it even harder. The lower the stump is, the harder it is to get out. A dozer isn't the best machine for stump removal in the first place, so having a small dozer without any power is just going to make it that much harder. What will really suck is that the blade will hit those stumps and stop you dead in your tracks. The first time it happens, you think, ok, I'll remember it's there and work around it. But very quickly, you will find that they are everywhere and you can't remember where they are or be able to go around them. It realy becomes extremly frustrating.
My dozer is a Case 1550, which is kind of in between a D6 and a D5 at 169hp. It will take out some stumps, but not very quickly or easily. My backhoe does a much job and it's faster too, unless it's just a small stump.
Your ideal machine is a trackhoe with a thumb. Something that can dig down a bit on either side of the stump and then pull it out. With this and at least 100 hp, you should be able to make some progress. More HP, the faster it will go.
For the burn piles, a dozer with a rake on it in the 200 hp range would be ideal. A Cat D6, a Deere 850, a Case 1850 and so on would be ideal. A little smaller will do the job, but just take longer. More HP would mean getting more done and getting it done faster.
With that much land to clean up, you will need allot of power and a dealer that is close by. You will break and tear up anything that you have when you do this much land. It's a huge job and I'd expect to spend five years on it if you are doing it yourself. I spent most of a summer, working 7 days a week with my dad helping to clear 7 acres of heavy timber for my pond. We took out every single tree and stump, then burned it all in one, long burn pile.
Another option would be to get a backoe with a grapple on the front. You can dig out the stumps with the backhoe, and carry the branches to a burn pile.
One big problem with pushing branches into a burn pile is all the dirt that goes with them. It might not look like much at first, but that seven acre burn pile ended up wiht hundreds of yards of dirt and ash after it was done burning. I've found that the dozer is terrible for this and I now just about refuse to use mine for pushing branches into a burn pile. It just takes too long to clean up and get rid of all that dirt after it's done burning, plus it dosn't burn so very good.
Around here, allot of people negotiate with the loggers to clean up and remove the stumps when they are done logging. Usually the price to clean up a logged site is almost exactly the same as the value of the logs. With the drop in log values lately, it probably cost more to have it cleaned up then what you get for the logs and they might refuse to do it, but the people I know who had them clean up the land said it was worth it. You don't make any money on it, but you get clean pasture to work with when they are done.
Good luck,
Eddie