In meiosis I, chromosomes are supposed to be matched up so that homologs can exchange DNA by crossing over. A 3n banana would have an extra set of chromosomes with no pairing possible. This would either stop meiosis there or lead to gametes with highly variable and inappropriate numbers of chromosomes. In either case, this would lead to infertility.
Work Step by Step
First, notice that 3n is abnormal. Then look at meiosis in Ch. 11 and note that the cells undergoing it are 2n. Imagine various ways in which this wrong number of chromosomes could disrupt gamete formation, leading to infertility.