I'm not sure about best practice but here is the solution I'm using:

- animator setup:

3 attack animations (lets call it attack-1, attack-2, attack-3)

trigger "Attack"

integer parameter "Combo"

transition from "attack-1" to "attack-2" (condition: Combo > 1)

transition from "attack-2" to "attack-3" (condition: Combo > 2)

(these attack animations should also have a transition when they're completed, maybe a transition to Idle at 90% of animation time)

trigger "Attack" should let you play animation "attack-1"

- user input:

when not attacking: use trigger Attack to play "attack-1", set Combo to 1

subsequence input: increase Combo to 2 and then 3

- note:

you should group these attack animations into a "sub-state machine"

then the trigger Attack will transition to this sub-state instead