With orders of $25 or more, Amazon offers Prime members Wilson's Championship High Altitude Tennis Ball 3-Pack for $2.97 with free shipping. (Non-members can spend $49 to bag free shipping.) That's the lowest total price we could find for a Wilson's 3-pack for by $4, excluding the mention below.

Prefer the regular altitude balls? eSportsOnline via eBay offers the Wilson's Championship Extra Duty Tennis Ball 3-Pack for $4.39 with free shipping.