I think you're gona have a hard time getting an ANTEC to run much over 12v. They are very good PSU's and are very accurate, it's part of the mfg process of them. A 5 ohm load should draw about 1A, which will definitely get hot, that's about 5.3 watts TDP, and the thing is only rated for 10w at something like 220C. The way to get around this is to put them in series, so you have 2 5 ohm resistors for a total of 10 ohms, and then you can run two of those setups in parallel for a net resistance of 5 ohms and a TDP that is the same 5.3w but divided between 4 resistors, or about 1.32w each which will run alot cooler.

if you want to go to 3 ohms total, there are many ways to get there, but one single 3 ohm resistor is not gona work unless you want it to get really hot

the reason for it not getting to 12.6v is your charger rather than the PSU. make sure to balance the packs, and really anything over about 12.4 will work, the difference between 12.4 and 12.6 is only about 3 to 5% on most packs.