(If anyone there now was responsible years ago for telling schools not to teach grammar anymore, I bet they regret it… 🙂 )
Spell out the number: three, not 3.
The acronym for unidentified flying object should be written in capital letters (the way acronyms generally are).
Do not use an apostrophe to make a plural. (There are rare exceptions to this rule, but the example does not contain one of those exceptions.)
When the common noun street is part of the proper name of a place, capitalize it.
There were three UFOs parked above Downing Street.