Growing multiples of variable combinations
You can think of these combinations as 3-digit numbers in base $l+1$. For example, if $l=9$ then the solutions to your problem are in one-to-one correspondence with 3-digit decimal numbers, from 000 to 999. There are in total $(l+1)^3$ of them.

As an aside, it is not clear why you omitted 0a0b0c.