Their lexical categories all differ!
That is common in Chinese, especially in Classical Chinese.
How did 3 semantically shift to 1, 2?
Sometimes the different meanings of a character are not evolved from the same origin. In ancient times, people often designated a character with a certain meaning merely for the reason that it shares the sound of the oral language of that meaning, and so is the case.
By the way, meaning 1 & 2 were fixed over 2200 years ago (before Qin Dynasty), and there are some other characters that is interchageable with 惟 under certain meanings (listed in bold font).
Let me list a relatively comprehensive entry of 惟（meanings still common in modern Chinese will be intalicized）：
唯：顺遂，听从（to obey, to submit）
希望，愿请（hope sb. to do sth.）