There are numbers...just not anything approaching numbers like 3000.
People like Muhammad Azeem, Abdullahi Omar Fidse, and Mukhtar Ahmad, all on the terrorist watch list, have been apprehended at the Mexican border. There's a small number of actual known affiliates and subjects on the watch list who have been detained on our Southwestern border since '01, but not enough in my opinion to make the wall about terrorist threats.
The wall is about Mexicans, I think Trump is just playing a shell game.
I will stand corrected, to a point. I think the sort of realistic number of people with "with ties to terrorism"
is something like 15 since 2001... and they were at ports of entry. Most, if not all, were during the Bush or Obama years. Trump & Pence have floated the number "10", but their support of their claim is tenuous (IMO).
Note 1: These instances were at ports of entry.
Note 2: Anyone should feel free to correct me (a link would be nice, so I don't make future mistakes).
Note 3: I have been stopped at the U.S. border at least 4 times (the most recent time in 2016)... my name was/is apparently on a watch list.