Each individual will have their own preference for which days of the week these should be. Some may find Saturday/Sunday/Wednesday convenient, those with family commitments might choose Sunday/Tuesday/Thursday. A great advantage of this routine is that one is getting ample recovery between sessions.
I would recommend a 'long' run each week with the other 2 sessions taken from the following pool of sessions:
- 6-8 x 400m with 2 minutes recovery
- 3-4 x 800m with 3 minutes recovery (this and the above session on a track or round a soccer pitch)
- 40 minutes fartlek (including warm up and cool down)
- Hill repetitions - 4 x 400m, 8 x 200m or 6 x 300m
- 2 x 2 miles with 2 minutes recovery
- 3 x 1.5 miles with 2 minutes recovery
- 40 minutes steady pace run
Choice from the above 'non-long run' sessions will ensure that there is quality work being carried out to maximise benefits from a restricted schedule.