What are the three alleles for blood and what do they do?

The three are Ia, Ib, Io.

They form one of six genotypes - AA, AO, BB, BO, AB, and OO and make up one of four phenotypes - A, B, AB, O.

They are antigen like protrusions on a RBC, and are most important when pertaining to blood transfusions, as this is the bodys signature, and different blood will set off an immune response

The A and B alleles code for enzymes that help put specific combinations of sugars on the surface of red blood cells.  The patterns of sugars are different for A and B.  The O allele codes for a nonfunctional enzyme, so people with O blood have none of these sugars on their red blood cells.  That's also why O is recessive.  These sugars don't seem to have any real purpose, other than helping our immune systems tell self from non-self.  There is some evidence that there may be an interaction between the sugars and the malaria parasite, but that's probably a side effect.

They're there to ensure your bloodtype.

Every human have a pair of two of them, and their fenotipe would have their bloodtype according to their parents allele crossing.

Simple example:

an AB blood married with OO blood. The possibilities of their children bt are : AO, AO, BO, BO because of the crossing of AB to OO, so the children must either have A or B bloodtype in 50:50 possibility, if the child bt is AB or O, then that's absolutely not their child.

Another pairing: AO x BO
crossing : AB, AO, BO, OO
so, the possibilities are: AB 25%, A 25%, B 25% and O 25%