Rids if you don't mind me asking, what is your current weight and estimate body fat?
You say you are going on a calorie deficet, so I assume you are trying to burn fat and hold muscle? If I am wrong let me know, but going on the sounds of it I would just throw in a power HIT routine. Go heavy, but on a weight where you can do a comfortable 12-15 reps. Try and super set everything you can and only have a 30 second rest period. You want to keep the heart rate up so that it is constanstly in fat burning mode. Also make sure the diet is on point for those 3 weeks and you can acheive a lot more than you think.
If you are happy to buy and take it, get some animal cuts as that will last you 3 weeks and with the right diet and traning can cut you up quite nicely.
On one of the days throw in a giant set by doing 6-8 exercises without a break. I know this isn't always feasible due to equiptment being in use, but if it's available give it a go. Example - Close grip chinups into wide grip chinups into seated rows into straight arm lat push down into lat pull downs into straight pull downs, into behind head reverse pull downs. 90 second rest repeat again.
Cardio (the word people hate
) Do one of the following twice a week at end of session:
30 mins high stepper:
2 min low resistance
2 min medium resistance
1 min high resistance
repeaet 6 timrs
30 mins walk on incline
2 mins speed 5 incline 5
2 mins speed 6 incline 10
1 min speed 7 incline 15
repeat 6 times (no holding on)
30 mins treadmill sprints
3 mins fast walk
1 min sprint
repeat 8 times
30 mins steady state
standard job for 30 mins