It splits into 3 pieces actually.
The other Cybran units that do this are: "TML-4" (the tech 2 building that launches TMLs) and "Plan B" (the nuke sub, which has a regular tactical missile launcher in addition to the nuke launcher)
If you want to count the exact damage, you can set up a custom match with cheats turned on. Go up against an AI:Easy. Then use alt-f2 to spawn things in for yourself and for them. Like you can give yourself a viper and give them a single TMD. If you spawn in some buildings various distances away from the TMD you can test it until your viper shot splits in 3, 2 of them gets zapped, only 1 gets through, and you can count how much damage the single projectile does.
You can also observe whether it looks like the mini-missiles are slowing down compared to the original missile. There is definitely no reduction in range because they go after the same target as before. I haven't noticed a difference in speed but I haven't looked closely at it.