I suggest the following:
1) Do 1 case from mck website in 60 min. Check your score. Target score is 22 out of 26 correct. If you do 17-18 correct you have great chances to improve in 3 days. If lower - reschedule the test.
2) Buy Viktor Cheng test prep program - best materials I've seen so far. Works also for express prep.
3) Do the 2nd test and check the score
4) Use other materials: GMAT IR and verbal parts, PST-like tests available online. The key thing - you should train to do them, fast
5) Do the 3rd test and check the results
P.s. in parallel train your math. Check exercises on Cheng website. Key things - multiplication of 2 digit numbers, operations with zeros and division (Learn division table, ie. 1/8 = 12.5%, 8/9 = 88%, Learn up to 8/9)