If you really need to translate "三字头", it means "it starts with the number 3", "the initial numeral is 3". "见"， originally meaning "witness (v.)", ultimately means "reach" here, or in colloquial terms, "hit". You have finally hit the 30s. 另一个例子: 他的手机号是9字头的。 又例: 股市能再见4字头么？(Will the stock index hit 4000 again?)

I think no,here 头 means 'start' or 'too' means it start from 3.20's just start 2.so 见三字头了means already seen the top of 3 Feel free to contact,I'd love to make friend with ppl who is interested in Chinese,btw,I am Chinese speaker

You get it right. The whole sentence may be translated as ‘Finally (be) in your 30s. Brother. 3 (comes) first in (*) order, finally. Happy birthday.’. I find some other exmaples on Bing.com. 从昨日开始，合肥气温悄然回升，最高气温今日将重回“三字头” the highest temperature is above 30 again. 天津房价迎来“三字头”时代

Yes you got it right sometimes 頭=首 (start) in Chinese for example : 開頭(beginning) (影片的開頭/beginning of the film) 出頭(mainly describing age) (他已經30 出頭了/he's in early 30s) we will use 尾to mean last as well eg :從頭到尾=from start to finish I am not an expert in Chinese teaching but hope it would be useful :)

三字头，不是三字的开端。是以3打头。means some word with 3 as the First character. 二字头，二打头。means it's with 2 as the first Character。 30岁，3 is the first word. just mentioning already in the 30s, so not young anymore. at least in China, we only call those under 30s, as young. if someone of your same age told you you are young, it's not mannerly proper.