Of coz they dunno what is mini sim as mini sim u mention is actually to be called "Micro Sim"(which is using on current HTC One, Samsung S5, Sony Z2 and previous Iphone 4/4S. the micro sim u tot is actually called "Nano Sim" which is currently only use on iphone 5/5S.

And for Mi3, in the phone it provide standard sim tray only. But xiaomi know that nowadays many ppl use "Micro Sim" already so they free us a Micro Sim tray as well. so you wont have any problem on fitting your Sim card into Mi3. For those using "Nano Sim" card like me then we have to buy Nano Sim to Micro Sim adapter which available in Garage Sales with price of RM8 only.