if you build 3 trns for UK 2, then your battleship can protect the trns and will not make it viable for germany to attack. if your attack does fail the US can always finish the job.
Unless you are planning on having the US navy consolidate with your 1 bb, 3 trn, this is probably not a good idea. We are assuming the GER air remains intact. 1 bmb 6 fgt vs. 3 trn 1 bb -> GER only loses 2 fgt to squash your navy. If US does consolidate in SZ7 to protect UK navy (this is on US2), they would only be able to do so with starting navy of 1 des, 2 trn (any US1 builds can’t reach there yet). And these trn would have to offload onto UK (they are not going to take WEU, and can’t unload to anywhere else) so Allies are still not landing anything into Europe or Africa until round 3 earliest.