Can 我 be used as: our, us, we?
If I want to emphasize it's my(singular) company, how to phrase My company in Chinese?

You can use the possessive marker 的 to denote relating to or possession of an object noun.
我的公司 = my company (you don't own it, just work for it, so  的 indicates 'relating', not 'possessing')
我的公司 = my company (if you own it, 的 indicates possessing' not 'relating')
This means either you own it or not, you still say 我的公司
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我公司 can be the shortened form of 我的公司 (my company) or 我們的公司 (our company) That mean 我 before a noun without a possessive marker 的 can be singular or plural
You can omit the possessive marker 的 and say 我公司 when you don't directly possess it, only relatively possess it. e.g. 我的父親 (my father) can be reduced to 我父親 because you don't own your father; 我的房子 cannot be reduced to 我房子 because you do own your house.
Also, check Why there is no 的 after 你 in 你国家的医院好吗?