Halliday & Matthiessen (2004: 344):
System III is the tense system available in non-finite and in modalised forms of the verbal group. Here a further neutralisation takes place, that is, both that in system II (affecting the past) and a parallel one affecting the future. … What happens here is that (i) past, past in present and past in past are all represented by past; (ii) future, future in present and future in future are all represented by future. There are twelve such triads; the total number of tenses in System III is therefore 36 – (2 x 12) = 12.