If you want to view an AH2 skin in AH3, you will have to rename and supply files or the skin is not going to look right.
P51D.bmp stays the same.
P51D_B_A.bmp -> rename <- P51D_S.bmp
P51D_B.bmp -> convert to normal map -< P51D_N.bmp (use the tools Greebo supplied links to)
The _B file is not used in AH3.
I have attached a split screen view of one of your skins in AH2. The left side shows what the skin would like copied directly over to AH3. The right side shows the conversion with the _B (bump map) converted to an _N (normal map) and the _B_A file renamed to _S.
None of the *.txt files are used in AH3. They have all be converted to image files with the channels of the image files containing data. That is where the _E and _P files come in.
Everything done in AH3 is to allow the skinner more control over the appearance of skins in the game.
Will your AH2 skin look exactly like it does in AH2, when it is converted to AH3? I don't know yet. We just did our first batch of skins. Take a look at the same P51D skin when it is down loaded in AH3 and you tell us.