This is a spin on the typical melee assist train group. Here's the template of the 3T Group again.
DPS Healer or another DPS
For the split assist, you are going to be in two teams.
Team 1: 2 suicide tanks, DPS, healer
Team 2: DPS tank and the DPS Healer or second DPS
Team 1's job to deal with the front line. Team 2's job is to push the back line and tie up the enemy healers or stay on the front line and simply hit a different target than Team 1.
The idea is that you want to force the single target healing of the enemy onto one target while the other team kills another target. Most POGs use a group healer (usually DoK/WP) and a single target healer (usually AM/RP/Shaman/Zealot). The PK bomb group sought to neutralize single target healing with AoE pressure damage by contrast.
The 3T split assist is more efficient than the PK bomb group, but it is more difficult to execute. You now become subject to damage spread where that was not an issue for you in the PK bomb group. You want to hit everyone you can in the PK bomb group. That's not the case here.
In order to beat this group, you need to first realize that you are getting the 3T split assist done to you. Team 2 often works the flanks, so if that's what they are doing, you can CC Team 1 and take out Team 2 quickly. Also realize that the non-tanks are all guarded by tanks. Tanks that are likely to be running Focused Offense. You have to neutralize the tanks to beat this group. That's one reason this group is so effective. People have ingrained in their heads not to beat on tanks when that's exactly what you have to do to beat this group.
Also realize that this group only has one real healer. That healer is well protected, but you can completely stop their healing with a single stagger or silence. You'll still have to deal with 3 tanks reducing your outgoing damage a lot, but at least they won't be getting heals for a little while.
Next time, I'll go over another way to play the 3T group. It's a bit more risky, but it kills stuff fast.