Think of #3 as a replenishment option. You may put down as many cards from your hand as you want into any number of already existing fields you control. Thus if you have a flax, onion, and castor field you an put any quantity of those three types of cards from your hand into play on top of those existing fields.
This helps you make existing fields bigger so others can't steal them from you - but be careful, usually anything more than 4 is going to be a target of the Locust.
Thank you for that James