In our everyday conversation of text messages, we use 足, however, sometimes we use it incorrectly. For human being, 脚 means "leg(s)" and 足 means "foot/feet". 脚 is also used for table, chair, etc. 肢 isn't read あし but し, and it means "limb(s)" according to my dictionary. (I didn't know its Englush word.) It's often used with other kanji, for exampke, for human beings 上肢、下肢 and for animals 前肢、後肢 (but I usually use 前足、後ろ足 for animals). Espero que entiendas. Un saludo.

：肢 limb http://www.thefreedictionary.com/limb ：脚 leg http://www.thefreedictionary.com/leg ：足 foot http://www.thefreedictionary.com/foot Hope this helps