[amibroker] How To Code "...three or fewer bars ago"?

[longarm61]

Sorry for yet another newb question.  Does the following code mean 
"buy if cross occurs 3 bars ago ONLY" or does it mean "buy if cross 
occurs 3 OR FEWER bars ago?"  I would like it to state the latter.  
How is this done if not with the code below?:

Buy =   Close > Ref( MA(  Close , 15 ) , -3 );