Frozen sperm. Fresh eggs. Both proven donors. Nulligravid 42 yo woman. Odds of conception after et with 3?

Very good. The odds are excellent as long as all other aspects of your workup have been negative. Pregnancy rate with 2 embryos can give a 70% chance. Replacing 3 embryos in this situation would not be suggested as long as the egg donor was 21-28. Most donors are not accepted over 30. The risk of triplets is quite high and would cause significant obstetrical issues such as prematurity, decreased fetal size..
With her own egg? If it's her own egg, the average likelihood of delivering a baby is just under 17% (from national ivf stats at https://www.Sartcorsonline.Com/rptcsr_publicmultyear.Aspx?Clinicpkid=0).
Age of the donor? The biggest factor is the age of the donor. Using an anonymous or known egg donor under 30, with normal ovarian reserve screening, the success rate should be 75-80% or higher with transfer of two blastocysts, and lower with a single embryo transfer. If the donor is older then success rates fall. By et with 3, if you're referring to 3 embryos, this is unusual, unless a known donor is 38+.