you most likely had some antifreeze and oil in the cylinder's when you took it apart if you did not clean it really good. that would explain the white smoke after 3-4 minutes and it might clear up. but as in you did a compression test what did you get for all cylinders? is one or two low compared to the rest? did you do a crack test on the cylinder heads? Did you compare the head gaskets old and new to each other? Did you put it all back together right (double check)? is there excessive pressure in the coolant system? did you put the gaskets on right? some applications there is a left and right side depending on the vehicle you have(most do not matter left and right but not all and I don't know what year make and model you have and I can't look it up for you without that info. and sometimes on v6 and v8 motors the heads have a left and right as well again sometimes Not often. I do have a couple more questions but I am getting tired now but these questions as you go through them it should help you. good luck in finding out what is wrong with it but my last question is did you torque it to specs not guessing if it applies 90 degrees and did you use a torque wrench?

Does the exhaust smell rich? If the head gasket was changed but it still smokes and the spark plugs have oil then maybe the head is warped. You can get away with torquing it more than usual but look into the head. Take the head out and check it

You might have put the gaskets on the wrong side (this is commonly done) it won't seat right if the gaskets are on wrong. It sounds like you could have fried piston rings though.

I wonder if the heads were tested for cracks? The 4.3 is notorious for intake manifold cracks--did you inspect the manifold and replace those gaskets, too?

Something is definitely leaking. The leakdown test should have pinpointed something but if not pressurize the cooling system and see if that produces coolant your plugs or air bubbles in your radiator.

The engine was running good prior to changing the head gaskets. Then I changed the head gaskets on my 4.3 engine due to coolant in the oil. After reassembly and after about three or four minutes white, and oily smelling smoke comes out of the exhaust and the engine runs rough. The heads had a valve job done. Compression is good and leak down test checks good. All six plugs are fouled with burnt oil.