A SERM is a selective estrogen receptor modulator, basically it acts by binding (Estrogen Receptor) ER-alpha so that estrogen cannot bind an exert an effect, its effects on ER-beta are much less to enable estrogenic binding, as binding ER-beta has positive effects, as opposed to ER-alpha, which are bad...!!
When using nolva & Clomid for PCT, the idea is to block the estrogen receptor to make the body think none is being produced, because no estrogenic response is detected the body thinks there is no testosterone (as test converts to estrogen, remember), so it re-starts LH & FSH (luteinizing hormone & follicle stimulating hormone) this restarts testosterone production, which in turn converts to estrogen and the balance is restored....
(Let me know if you want the more detailed explanation, but that is very much the basics)...
If you have only been on Dbol 3 weeks, you do NOT need PCT.... I have posted previously on the negative effects of using potentially harmful anti-estrogenic drugs needlessly.. You dont need Adex (up to a week), Clomid (four weeks), overlapped with nolva (four weeks), then adex (few days)....... Just for 3 weeks on dbol.... Your body will return to normal, trust me...
Arimidex, is an AI (aromatase inhibitor)... Testosterone aromatizes into estrogen (thats how males get estrogen) via the aromatase enzyme... Arimidex lowers the amount of enzyme needed to convert testosterone into estrogen, therefore you end up with less estrogen in the body to exert potentially harmful effects.... The body does however need normal levels of estrogen for healthy function, so knocking out estrogen is definitely not advised... Arimidex is sometimes used on cycles that tend to cause more aromatization to occur, that raise estrogen to unhealthy levels, a little adex is used to lower the conversion process to normal levels... One thing with anti-estrogen drugs, you only use them if blood tests show elevated levels of estrogen along with common side effects of high E2....!!!! (I've tried to keep it simple, I hope you understand)....