I'd definitely choose the D3100. I don't understand the "purpose" question when you didn't give a specific purpose but the D3100 would be the better performer of the 3.
As for the difference between point and shoots and DSLRs, it'd take quite a bit to write and that's what a good google search is for. So here is one link that tells the difference between point and shoots and DSLRs.
I am Searching these days for entry level DSLR ..
after my search i have scene these three quite fine ..
which one would be best of following three
or is there any other entry level good for purposE ??
and there are also canon powershot available ie
Canon PowerShot SX30 IS
what is difference between dslr and power shoot