I am sure this was already discussed before but cant seem to find it.

I have tried to get the answer but different people have given me different answers.

I went to Penang for my non iim visa B and although my company requested a multiple entry visa, I only got a single entry but I heard that this is pretty common.

My company has applied for my work permit and will receive it soon.

The question is: Do I have to leave after 3 months or can I extend it somehow at immigration office?

The question is: Do I have to leave after 3 months or can I extend it somehow at immigration office?

yes you will have to leave on a visa run every 90 days unless you get an extension from thai immigration...here is link with more information

10 step instruction: One year work permit and visa

Upon the completion of your work permit, you can go to immigration to have your visa extended until the duration of your contract. But they will first give you a checklist of documents to submit before they give you one.

If your company isn't able to finish processing your work permit within 3 months, then you can go to immigration to apply for an extension while your work permit is still in process. From my experience, immigration gives you a 14-day extension first, and another 7 days thereafter.

Hope this helps.