Um... can Trunkten go SSJ2 and SSJ3? Because he should be able to... Even if not though, we've seen that the potara fusion is RIDICULOUSLY superior to the Metamoran fusion.
Base Vegetto was able to fight on par with Buuhan, while SSJ3 Gotenks was equal to BASE Super Buu. If Goku or Vegeta were a SSJ1 against SSJ3, or even just SSJ2 Goten or Trunks, they would get RAGE-STOMPED. So, we can infer that SSJ1 Gogeta would get RAGE-STOMPED by SSJ3 Gotenks. This would mean that Gogeta would at least have to be SSJ2 to be around equal to SSJ3 Gotenks. This alone proves that a hypothetical SSJ3 Gogeta would be weaker than SSJ1 Vegetto, or possibly even weaker than BASE Vegetto, which is honestly more likely.
So.. As strange as it sounds, Base Vegetto > SSJ3 Gogeta.
So... SSJ1 Trunkten >>> SSJ3 Gogeta. Since we can presumably put SSJ1 Trunkten at even greater than Base Vegetto, just from the fact that SSJ1 Trunks was actually able to contend with SSJ1 Vegeta.