100% YES!
The GMATPrep 3 and 4 are challenging and AWESOME, in my opinion. If your target score if 650, I'd suggest you attempt to score a 670+ on these practice CATs to have a comfortable margin. This is not easy. These CATs are HARD (and awesomely so).
If you have a 620 on the GMATPrep, you're probably still cruising around that score-range unless that CAT was a number of weeks ago. So you really need to make up those 50 points STAT, since you only have 3 weeks. It's do-able, but it will require some laser-like precision.
I'd immediately hit that GMATPrep and use it to complete an Error Log (see attached template), refine your Verbal strategies (especially RC -- most students tend to ignore it), and focus on 5-6 weaknesses each week. What's your Verbal/Quant breakdown? How well are you doing with SC concepts? Do you have the MGMAT SC?
Here's a post I wrote on breaking down GMAT RC:
https://gmatrockstar.com/2014/01/17/gmat ... y-purpose/
And incorporating alternate Quant techniques such as Picking Numbers:
https://gmatrockstar.com/2014/04/15/gmat ... the-day-2/
Good luck!