For everyone that reads my blog, I understand how busy we can all be. Just because you are unable to go to the gym 5-6 times a week DOESN'T mean you can't be SUPER fit and feel amazing about your body. As I have said numerous times, diet is key. If you only have the time a few days a week to train just make sure your diet is clean and I promise you will still be able to stay on track.
If you only have 3 days a week to train you can workout using a push/pull split ( one day chest, shoulders and triceps; back and biceps the next; and legs the last day). I am aware that not everyone loves fitness and or the gym as much as I, but that doesn't mean you should ignore your health all together. Get to the gym at least three days a week and make sure you are lifting weights.
While lifting you can start with 3 sets of 10 repetitions at about 80% effort, the last few repitions should be difficult and if they aren't you need to increase the weight being lifted. Take about 2 minutes rest in between each set. If you get used to 3x10 you can change it up to 3 sets of 12 repetitions at a lesser weight (about 70%). With the decreased weight you should also decrease your rest time (about 1 minute). Keep switching up your sets, reps, and rest so your body doesn't accomodate and hit a plateau. Try changing the number or reps by going heavier some days with less reps and lighter other days with higher reps.
After you are finished with your weights for the day, if you are in a hurry jump on the treadmill and get a quick HIIT workout in. They last only about 20 minutes but keep your body burning fat for 24 hours+. If you have a little more time hop on the stairmaster for 40 minutes (I know its killer but the results are worth it).
My advice if you only have those 3 days, HIT IT HARD. You will have plenty of recovery on your days off so you can go "BALLS TO THE WALLS" and sweat it out while you are there.
Don't hold back, and when you step in to the gym make sure you are "gettin it". No one matters but you once you are there; work on your goals and don't hold back! Keep your workouts INTENSE!