As said below, which is what we do...3 of x for $20, as a separate item entry, but we have to stress that the online customers have to tell us what colors they want...(this is for socks)  Currently there is no way to set up under an item category a way for a customer to select three different colored items of the  same category and be charged  the 3 for $20 price.  This is aggravating...

@malcolm, @VanKalkerFarms --- this is automatic no employee input you may want to try this?

I have this setup for my 16 oz candles...and it does it automatically at the POS.

1 16oz candle is $14.99

2 16oz candles are $24 dollars

3 16oz candles are$33 dollars

I also have for my 28oz jars that when you get 12-28oz jars you get a 13th free or a bakers dozen.

Here is how Square tells you how to setup Discounts:

create-and-manage-discounts 

Here is a Screen shot of my settings:

My settings for the 3 for $33

I deduct 3.99 from each jar to get to the $33 for 3.  Now each time I sell multiple of 3 16oz jars the customer gets the discount.  This is only for the category of 16 oz Jars.

This works for when people purchase different scents in our situation which should work for you with different colors of socks.