Are all the 1wg rear subframes the same, need one for an 3en2?

No..they are specific to the model.

Boy this is tricky, how can I tell a 3en2 one?

This is one;
http://www.ebay.co.uk/itm/Yamaha-FZR400 ... 3cf8c21b55

Check out the lugs and bracketry and note them for identification.

See the 2x very shiney square threaded mounts at the very rear....that is where the rear plastic joiner part fixes.
Between them is a boxed in shape ........the passenger seat when raised has a tang that locks into that.

Both are specific to the seat panels and catches.

There are loads of small pieces that make up the subframe assy and you could search for ages/lots of money for all the parts if yours is non std.

There is a parts manual in the docs. section of this forum....you would be advised to view the page with the subframe on to see just how much you are missing.

Looking at your other post I'd concentrate on getting it's running right before this. These bikes are £800 - 900 in good condition.
Right now a grand will buy a good 3tj too

If you are determined to reinstate the original subframe; you would be advised to contact a seller like this and work out a deal on the complete unit with the seat lock and both seats.
http://www.ebay.co.uk/itm/Yamaha-FZR400 ... 3aa82fe3b1

You will still need to get the plastics though.....and be forewarned the rear seat joiner is a hard to source item....
.........and the one behind the passenger seat is var nigh impossible.......clumsy folk tear them off and smash them or lose them somehow..

Or find a race seat

Bought the subframe and undertray bits :)