Not possible with NS 3 I am affraid.
To play on LANs with Steam-Based games you need to have first connected then closed steam without loging off. Then play as normal.
So if you cannot do this, you cannot play the LAN game.
However, it should be possible to dig up an old Ns2 installer and use that on a WONID installation. This does not require internet access. You can still use the new maps too, but various weapons/balance changes won't be there. It still makes for a good game though.
Note that the server is the only pc that needs the 2+ patches, and these are optional anyway. The biggest difference I remember is reducing rine rts from 20 to 15 res.
You should be able to find a copy of the NS installers, and you only really need one copy of half life between you on a LAN game. Of course, this is naughty, and you should feal guilty and go buy 6 copies.
Don't laugh. I did.

(Second hand off eBay for ?2 each. A cinch.)
Feel free to PM me with any questions.
BTW, if your friends are bringing their own pcs, make sure they know exactly whant they need to bring and what you provide.
Notes:
>Power adapters are like gold dust in lan parties; have spares at the host's house.
>Tell your friends to label any property of theirs that may become confused with someone else's.
>Dont bring speakers if you are all playing in the same room. Headsets cause much less confusion.
>CRT monitors dont lend to being dropped. Take Bubble wrap or use an LCD screen.
> LAN CARDS! Not all motherboards have them built in. Not much use having a PC without a NIC in LAN games.

>Allow at least one hour for setting up. Nothing works first time.
>Win98-WinXP on WonID lan games is a pain. Win98-win98 is ideal, if not, xp-xp. Do not allow windows ME pcs into the house. Just don't. It's morally wrong.
HTH!