Generally, [a.] would mean you took one bus, multiple times, and in a different context would be valid. like:
Twice last week, I ate a sandwich.
Tells me that there were 2 times last week that you ate 1 sandwich.
I've had tacos 3 times in the last month
Could mean you had one taco one three separate occasions, or had 6 tacos on each of three nights, or anything in between. Aside from detailing your exact dinner plans, this is the only way to express [3.]. It's also the only way I can think of to express [2.].
That said, both
I took buses full of students last week
I took a bus full of students last week
Sounds more like you're saying you literally stole the bus(es), and put them somewhere. You'd be understood, but only because people would assume you didn't mean that you physically stole them.
Rather, I'd say:
I took a really crowded bus twice last week.
(this would mean it isn't usually that crowded)
The buses I took last week were full of students
( this would mean that riding a bus isn't abnormal for me, but it was abnormal for there to be a bunch of students on it - usually it's non-students)
I took the bus last week, and it was simply packed.
I rode a bus full of students twice last week.
(this has the connotation that I don't usually ride buses with students, but last week I did twice)
If you want to emphasize the fact that it was full every time you took it, I'd say something like:
Both times I tried taking the bus last week, it was _____
('completely full', 'packed', 'crowded', 'like a can of sardines', or whatever you're trying to highlight).