I think this is in right section. Sorry if not

Been given a huawei 3 mifi dooberry on 3 network needs a SIM card can I use a std 3 3G one or is there a special one?.

3 have SIMs specifically set up for mi-fis and mobile broadband devices, you'd be best getting one of them - especially since it's against their terms and conditions for you to use a SIM on a phone tariff in a mobile broadband device.

You need a broadband sim / package to use in a dongle, not a phone one otherwise you'll see a tethering block page or risk it being blocked.

If it's for a lot of use then you'll need to get a 10 or 15GB a month deal or something.

If it's only for occasional use look at Samba, as they use the 3 network and it will work. It's ad funded mobile broadband.

Though that would require unlocking the mi-fi, would it not?

No as Samba is Three and it uses the same APN and settings. Any 3 dongle should work with Samba.

Cheers