They'll give me a lock for multi-acc?

I don't think so. Both me and my housemate has been using SteamGifts (different accounts, of course) for a couple of years using the same wifi and haven't had any problem since.

Send support a ticket explaining the situation then they should give an okay.

You are in the building?
Who else is there? ;)
Who are gonna use the accounts, for me it's not that much of info to hear you are using a router & 3 devices?
Will you just try to make the accounts from different ip's or what do you mean by that? :)

Believe me, most of the times it is really obvious if an account is some alt/smurf of someone or not. Most of those people don't care to maks it (or not smart enough to know how to).