the rate for the first 3 minutes is a, since it's a flat-fixed value, thus the cost will be a.
for Jackson, t > 3, namely 4 and higher, if he's on the phone longer than 3 minutes say hmmm 4 minutes, the the last minute is at a rate r, and his cost will be 3a + r1. now let's check a table.
4 minute..................a + r1...........3a + r(4 - 3)
5 minutes...............a + r2...........3a + r(5 - 3)
6 minutes...............a + r3...........3a + r(6 - 3)
t minutes................a + r(t-3).