I used clomid in the past & between 19mm & 23mm a follicle can rupture for ovulation. 3mm lining is quite thin but did you know that clomid does actually thin the uterine lining. It did this with me & it also made my cm thicker so its harder for the sperm to swim through plus after 6mths of clomid i never got my bfp so switched to tamoxifen & my lining was thicker 10mm+ on like cd10 & i got my bfp on my 2nd go of tamoxifen.
When your on the likes of clomid or tamoxifen these types of fertility meds shorten your cycle length to 28 days ( Most of the time ) it is unsuall to see a lining of 3mm on CD9 - it should be around 6/7mm by this time in the cycle. However your uterine lining grows 2mm per day so there is still time for your lining to catch up because If your biggest follicle was 11mm & a follicle grows 2 mm per day then you prob would ovulate around CD 14/15 & your lining had loads of time to catch up if it grows 2mm per day hun
As for your scans, you should have a scan while on fertility meds incase you do over stimulate & develope ohss ovary hyper stimulation syndrome. Its not unusal for these doctors to try & fob you off & not give you a scan because all they think about is money. They tried it with me onec & i spoke to my gp who wrote to the doctor & explained about how my health while taking fertility meds should be taking serioucly & i should be monitored ( which they did) in the end.
Goodluck hun x