There are two possible translations to this sentence:-
'There are three socks here' or 'Three socks are here'
Usually 'mae' without a following definite article, pronoun, noun or proper name is translated as 'there is/are', otherwise the meaning of 'mae' = is/are
The second point you make is about 'socks'
A sock = hosan; Socks = hosanau (but this is often contracted to 'sanau')
Numbers take the singular noun in Welsh so 'three socks' = tair hosan (because 'hosan' is a feminine noun)
You have to remember that Cymraeg doesn't work off of the same gramatical system as English. If you think back to the previous subject where Mae was introduced, it was a positive confirmation as opposed to a question or a negative.
In english you'd have to translate it according to the context of the sentence. The positive confirmation of is would be There is. However, since in this context it's plural, it would be There are.
That is my understanding of it anyway.