Hi im 38 weeks pregnant and I was using a dildo to masturbate and noticed in the toilet there was two big globs. Is it my mucus plug?

Yes. Yes, orgasm can cause uterine contractions so likely there was some disturbance in the cervix that caused this mucous to dislodge. No worries, everything will be fine.
Most likely. And since u have had 3 csections, do not rec doing that anymore as it could make your water break or put u into labor before your c-section date an I am sure both you and your OB would rather not do a 2 am c-section unless necessary.