Not sure if anyone had given you the answer you needed so here is the formula:

I'll denote capital X the amount you bet on horse X and little x the dividend (not odds) of that horse.

Let A = amount on horse A
Let B & C be the horses you want to save on and b and c their respective dividends.

Then B = (A * c) / ((b*c)-b-c)
And C = (A * b) / ((b*c)-b-c)

Now I'll apply this to a situation where you put $100 on Horse A, Horse B has dividend $10 and Horse C has dividend $6.

Amount on B = (100 * 6) / ((60)-10-6)
= 600 / 44
= $13.64 rounded

Amount on C = (100 * 10) / ((60)-10-6)
= 1000 / 44
= $22.73 rounded

So the total outlay is $136.37 and the return if B wins is $136.40 and if C wins $136.38. The difference between the outlay and return is due to rounding.

I hope this helps.