That would depend on both the individual and combined weight of the items. 

For example, the minimum cost to send a letter (postage cost only) is $1.40 -- x 3 would put the postage cost at $4.20. Your packaging costs will be x 3 as well, so add that on.

If the packages are in the next weight bracket 125g+, then that jumps to $2.10 x 3 ($6.30). 

If all three together weigh less than 500g, a parcel will cost $7.15 (if done through eBay), plus 1 lot of packaging, so $1-$3 more in postage, but you'll also have tracking and proof of postage. 

Edited to add: if lowest postage is the primary consideration, you can get two standard cases [side by side] in a larger envelope, so might be able to reduce total cost by sending 1 on its own, and 2 together.