I don't think it really matters too much when you do it. I personally prefer conditioning on non-lifting days but everyone's got their preference.
I was really thinking more along the lines of lifting 3x week, cardio 3x week. you could do 3 main lifts a week and have the "three week cycle" extended to four weeks. or you could combine the press & deadlift.
Though I have to ask, how much swimming have you been doing up to this point. 7.5 km/week seems like a ton if you're not an avid swimmer already.
EDIT: If your not specifically training for distance, there's probably better ways to go about it. for instance:
Day 1: lift
Day 2: Continuous high-intensity (build up to a 10-15 minute spurt at the max intensity you can sustain for that duration.)
Day 3: Lift
Day 4: Long, slow distance(long swim at a relatively easy pace)
Day 5: lift
Day 6: HIIT intervals ( swim 100m, rest 1-2x the amount of time the round took, then repeat)
Day 7: off
just add 1 minute to day two, 100 meters to day 4, and 1 more interval to day 6 every week minus the deload weeks. And start low, like 10 mins CHI, 1000m LSD, 5 HIIT