The only difference between the 2100, and the 2105 is the embedded graphic's part. 2105 having the better gpu of the two.
The i3-540 is the oldest of them all, and have a different socket (lga1156), so unless you were thinking of using them with different motherboards, you can't use it with the same MB, as with the other 2. (apart from being the oldest, it is also the lesser choice of them all, given the DMI speed, only performing half the speed of the others - DMI explained here)
So, to sum it up a bit.
If you are going to use the embedded graphic's chip, the 2105 will be my advise.
If you are using another gpu anyway, then you might want to save a few $$, and buy the i3-2100 cpu.
The i3-540 can only be attractive imo, if you already have a mb, for it (assuming you otherwise would have to buy a new one), and are using an external well performing graphic's card. Otherwise I recommend you avoid it (the cpu socket is obsolete - lga1155 is the "new" socket - for now :D ).