How late can implantation happen after 3dt. My 1st beta was 10 (11dp3dt). 2nd beta in 2 days.?

6-7 days. All embryos always implant between 6 and 7 days of age, regardless of day of transfer, and regardless of assisted vs. Natural conception.
Around Day 7, varies. With day 3 transfer the embryos have to reach the blastocyst stage, around day 5, and usually implant around day 7. Some embryos don't get to the blastocyst stage until day 6 or day 7, so they could implant a day or two later than average. 10 is a positive first beta HCG level (pregnancy hormone level), but may be a little low. The second beta HCG should go up at least 50% over 2 days. Best wishes.