>Pretty sure any printing site will do that automatically. I'm more worried about aspect ratio.
No it is not possible for them to add the bleed... (that requires image data outside the standard A4 area) all they could do is expand the standard A4 page by say 104%... if this is not designed into the layout then things like page numbers and edges can get cut or become visually close to the edge of page.
The final trim size is designed for A4
printed at 100%. The printers should be able to add any binding area accordingly (should it be spiral bound). But any extra area added over 3mm will be blank!
The "normal" no bleed issue 3 is best set for screen and home printing (i.e. where you will not print over size and then trim down to A4/3).
Best way to do this zine is to impose pages (printers can do this bit) with bleed to double sided SRA3 and then trim, fold, staple the spine - Tidy!
Pages on 120gsm matte
Cover/rear on 270gsm Gloss
Zines 1+2 may not quite setup the same way?