I have a similar setup on most classes. I'm pretty sure with couch_sitter's suggestion you'd be constantly attacking, but I haven't tested it. This is how I would do it:

that makes me switch to melee then nonstop attack i want to hit mouse2 again after switching to attack

alias melee "slot3;attack_bind"
alias attack_bind "bind mouse2 +attack"
bind mouse2 melee
alias meleerebind "bind mouse2 melee"
bind 1 "slot1;meleerebind"
bind 2 "slot2;meleerebind"

should work, didn't test it

alias +melee "slot3;+attack"
alias -melee "-attack"
bind mouse2 +melee

pretty sure everything above is incorrect

alias +swapmelee "slot3"
alias -swapmelee "-attack; bind mouse2 +meleeattack"
alias +meleeattack "+attack"
alias -meleeattack "-attack; bind mouse2 +swapmelee"

bind mouse2 +swapmelee
bind mouse1 "+attack; bind mouse 2 +swapmelee"

the issue with this bind is that its awful because you have to always hit mouse2 to swap to melee weapon before you can attack with it,

also you have to hold down mouse2 if you want to continue to attack otherwise you have to double tap it.

it's actually an awful idea even if you can get it to work. why cant you mouse 2 to melee and mouse 1 to attack, like what is your mouse1 doing this whole time

i feel like i just got trolled

smh

using binds in your aliases
shiggy diggy

actually just do

alias +swapmelee "slot3"
alias -swapmelee "bind mouse2 +attack"

bind mouse2 +swapmelee

bind 1 "slot1; bind mouse2 +swapmelee"
bind 2 "slot2: bind mouse2 +swapmelee"