First of all, I wanted to clarify from before that I have never gone back to the "heavy duty" style of training since I switched. I don't want to turn this into a Mentzer bashing thread because there are anough of those around.
I think you can get a more effective workout in 30-45 minutes 3x/week if you don't use HD. You will have to get out of the mentality of "complete and total recovery between workouts" if you got to that point, and you also have to avoid getting totally psyched up for sets.
Within your parameters here's what I would recommend based on what has worked for me:
Day 1: Lighter/Higher reps (12-20 reps)
Day 2: medium (6-10 reps)
Day 3: Heavy (3-5 reps)
Basically day one you use about your 20 rep max or a little heavier, day two you use about your 10 rep max or a little heavier and day 3 you use your 5 rep max. Days off should go between so you are not training on consecutive days.
Each day you should do 1 big pressing movement, 1 big upper body pulling movement and at least 1 lower body exercise. Do 3 sets all at the same weight for these exercises, gunning for your rep goal (20, 10 or 5) on the first set. Don't force any reps. Don't worry if you don't up your reps each week because you are going to be adding sets. In addition to the three big exercises, you can add 2-3 smaller exercises, but set a 45 minute time limit and try to pace yourself and save about 5 minutes at the end "in the tank" so you know you could get more next time. Use different exercises on each day of the week.
On week 2 add 1 set of everything. On week 3 add another set of everything but still trying to get done in the time frame alotted.
Week 4, 5 and 6 cut to just 2 sets of everthing but you can throw in a forced rep, or a drop set, or something to increase the challenge.
Example routine with possible rep numbers:
Day 1: all sets done with a 20 rep max or a little heavier.
1-leg squat 3 sets
Dips 3 sets
Shrugs or Upright rows 3 sets
Day 2: 10 rep max 3 sets each
Stiff leg deadlift or type of goodmorning
Dumbell Incline press
Cable or dumbell rows
Day 3: 5 rep max 3 sets each
Chin-up (ie underhand)
week 2, do 4 sets of everything. Week 3 do 5 sets of everything. You should be able to do this in 30 minutes. Don't add weight over the 3 week period unless you are way off in your estimate. In the final week, your reps on day 1 might look like 20, 16, 12, 10, 8. Day 2 like 10, 8, 7, 6, 6
and day 3 like 5, 5, 5, 4, 4.
Now for weeks 4-6, cut to 2 sets, add 10% weight, push a little harder, rest a little more between sets, even feel free to draw the 9 workouts out over 4 weeks if you want extra days off. This would give you a 7 week routine.
If you want to repeat the program, switch up your exercises somewhat.