I recently got my non-im B visa in Malaysia, and subesequently got my work permit back in Phuket. However, the visa was for only three months, but now I have a 12-month work permit. What do I need to do to extend the non-im B visa to match the work permit? A friend in a smilar situation had his visa extended, but he can't remember what paperwork he needed. Can anyone help me please?

What paperwork you need will depend on your work. Normally employeer has someone to help with that issue. You can find general information on the immigration web site for various occupations but it may not be completly up to date. If a new company they will unlikely be able to provide tax records for first year yet and that is often required. If unable extend you have to exit to obatain a new visa every 90 days.

http://www.immigration.go.th/nov2004/en/base.php

"However, the visa was for only three months"

Is this correct? I suspect your visa was issued by the thai consulate for 12 months but you have a 90 day 'permission to stay' stamp in your passport. You can change your NonB visa to 12 months at immigration IF your thai company/employer employs a minimum of 4 thais for each foreigner. Otherwise you will have to do border run every 90 days. If you do border run just before your 12 month visa expires you will get another 90 day permission to stay and can then obtain a new visa when that permission to stay stamp expires (ie, you get 15 months off the visa).

"What do I need to do to extend the non-im B visa to match the work permit?"

WP expiry date used to be aligned to the 'permission to stay' stamp in your passport. That meant renewing your WP every 90 days if on 90 day permission to stay. But that rule changed earlier this year. WP's are now issued for 12 months .... one sensible thing in LOS :-)