It can get frustrating when you can’t manage to get past a certain weight on a given exercise. Thankfully, simply increasing the weight on the bar isn’t the only way to progress and get stronger.
Performing more reps with a given weight is an easy progression to track. For beginners and even intermediate lifters, a simple linear progression would be a good place to start with a weight you can easily perform 3 sets of 8 reps with. Each session, add on one rep, so the next session would be 3x9 and then 3x10, up until you can perform 3 sets of 12 reps. Then simply increase the weight and start back with 3 sets of 8 reps. Similarly, when it comes to sets, changing a 3x6 to something like a 4x5 or even a 4x6 if you can manage it, is a simple way to get in some extra volume, which we know is key for hypertrophy (Schoenfeld 2012)
Although, longer rest times of 3-5mins seem to be necessary to preserve maximal strength throughout sets (de Salles et al., 2009), when training for hypertrophy decreasing rest times can be a handy way to increase metabolic stress (Goto et.al., 2004), another driver of muscle growth (Schoenfeld 2012). If you performed 3 sets of 12 at a given weight last week and rested 2 mins between sets and this week rested 90secs between sets, that’s a good indicator that you may be progressing. Of course, this doesn’t mean keep on decreasing rest times, just realise that if you rest slightly less, that is a progression
Finally, increasing the time spent in the eccentric part of the lift. Focussing solely on concentric contractions means you're missing out on 50% of the lift each rep. As well as this, eccentric training is better than concentric at building both strength and muscle & is necessary for maximal hypertrophy (Roig et al 2009, Hather et al 1991)