Hi there.
They definitely will grow a bit between now and ovulation. Sounds like maybe the RE wants to target the 3 at 15,,, i know at our clinic if we get too many mature follicles they will cancel the IUI, because the danger of high order multiples becomes too great. The trigger will help the 15's to mature too.
Wishing you the best of luck,
g
It depends on what meds you used. for clomid or femara I think they should be around 20mm. For FSH drugs around 16-18mm. but remember they can grow 1-2mm a day so a 15mm at trigger could be 17mm by ovulation. I took a combo of tamoxifen and follistim for our first four rounds of IUI and each time my follicles were bigger than 20mm. but this time with follistim only I had one at 15mm and one at 13mm. I also have PCOS and am currently 6DPO. good luck!