Triceps, when properly weight-trained, make up two-thirds of your upper arm's overall size. As such, it's better to do a quick tricep workout than to skip it when your gym time is tight. If it's absolutely necessary to skip a bodypart when training arms, better you should skip biceps than triceps if you're looking to develop upper arm muscle mass.
Your triceps brachii muscles have 3 heads - the long head, medial head and lateral head. They run down the back of your upper arm, from shoulder to elbow, and are mostly involved in extending your elbow to straighten your arm. The concentric portion of each rep contracts your triceps and fully straightens your arms, while the eccentric portion lowers the weight by allowing the elbow to bend to 90 degrees or more.
While twice the size of your biceps, the triceps are still one of the smaller bodyparts and can recover more quickly that larger muscle groups like legs, back and chest. This means you can work your triceps 2 - 3 times per week on non-consecutive days, but be careful not to use heavier weights than you can manage with proper form to prevent damaging your elbow ligaments or joint.
If you're training time is limited during a workout, use these three exercises to work the triceps, doing 3 or 4 sets of 10 reps each. Don't rush the reps - use a cadence of 1 second up and 3 seconds down with no pause or rest at the top or bottom of each movement.
Lying Triceps Extensions
Also known as 'skullcrushers', lying triceps extensions start with you lying flat on the bench with your arms straight up above you, perpendicular to your body. The exercise can be done with dumbbells, a straight bar, a cambered (EZ curl) bar or the specialized triceps bar. Start by slowly lowering the weight to your forehead then return it back to the top position at a faster speed. At the end of each set, go immediately into the next exercise with no break.
Narrow-Grip Bench Press
The second half of this triceps superset is the narrow-grip bench press. If you're using a triceps bar then you'll automatically have a narrow grip already, but if not then be sure your grip is about shoulder width or a touch narrower. Going any narrower than that places too much strain on the wrists at the bottom of the movement, so if you find your wrists getting sore widen your grip slightly. Since you're already lying on the bench in the proper position, lower the weights to your chest while keeping your elbows tucked close to your body, then raise the weight directly up by extending your arms. Do not let your elbows drift out to the sides as this involves the pectoral muscles of the chest more than you want it to - keep the tension on your triceps as the weight moves up and down.
Because you're supersetting this press with the extensions the weight won't be as heavy as you could do otherwise, so you can compensate by either doing a few more reps per set or slowing each rep down, especially on the eccentric (lowering) portion of each rep. At the end of your set rest for about a minute, then go back to the lying extensions to begin your next set. Perform the full cycle 3 - 4 times before moving on to the third exercise.
Overhead Triceps Extensions
Sit up on your bench with your feet flat on the floor and a neutral or slightly-arched lower back. Using two hands, suspend a dumbbell overhead so that it rests perpendicular to the floor - your thumbs and forefingers should be wrapped around the handle while your palms and other fingers are pressed against the underside of the top BELL to support the weight. Keeping your elbows facing forward and tucked close to the sides of your head, bend your elbows and slowly lower the weight as far down as you can behind your head, then return the weight back up to directly overhead. Your upper arms should not move during the exercise but instead stay upright and close to your head.
Other Triceps Exercises
The three exercises above will give you quick tricep workouts that are effective in building more muscle, but if you need to replace any of them due to injury or other physical issues, replace the necessary exercise with dumbbell kickbacks. Kickbacks have been shown to stimulate all 3 heads during each rep, so it's your best replacement exercise. Other triceps exercises like pushdowns, rope pushdowns, overhead cable extensions and triceps dips are good too, but save them for your workouts where time isn't limited and you can fit one or more of them in with these three.