To get an invisible patch, you need to cut out more plaster and rocklath (the 3/8" thick stuff under the plaster that looks like sheetrock.
When they plastered the two rooms, they applied varying thicknesses of material over the lath, so to try to reconcile the two ceiling levels within a scant 3" space is not going to be possible without making a big lump on the ceiling. i can already see how one ceiling was 1/8" lower that the other one.
I would pull off the expanded metal mesh, then trim back any subsequent damage to the rocklath so your patch is at least one joist bay in width. You should be able to apply a strip of 5/8" drywall in the gap if you cut back to a joist on either side, or run 1x3 furring strips every 2ft secured into the plaster with screws and some glue to the framing side of the rock lath. Shim the 5/8" drywall with cardboard strips so it is almost perfectly flush with the surface as best you can, since the original plaster varies. Then tape and mud it in.
I would treat the edges of the plaster with bonding agent (Link or Plasterweld) so the mud will stick to it; it doesn't do normally.

What I recommend only applies if you want a good-looking and lasting patch. If this is a rental or a flip, do whatever, screw in some 1x3's above, put in 1/2" drywall and 1/4" of mud, watch it crack.
Casey

What was the top of the wall connected to?

If the joists are perpendicular to the 3 in. slot just screw the new board onto the joists and span between them.

If the joists are parallel to the slot and not located over it, then get some strapping (say 9 in. long) and insert it into the slot, turn it perpendicular, and fasten through the existing ceiling on either side. Then attached your new patch board to the strapping.

Patch the attachment screw locations at the same time that you patch the ceiling.