I'm replacing the boards on my wood deck, as the old ones are rotten. The existing boards were attached with decking (spiral) nails. Each plank has 3 nails at 10 different spots. I'm assuming that if I just put the nails in where the old ones were, that it won't hold as well, since the holes will be looser. The placement of the 3 nails will make it difiicult to put three new nails through in differnet spots than the old ones, so I'm, wondering if 2 nails will hold well enough. Also wondering if regular nails will work, or if I need to use the spiral nails. Thanks in advance for your answer.

We Never Use Nails, For That Reason They Come Loose, Get You Some 2 1/2" Deck Screws And Put To In Each Joist. This Is The Best Way.
If You Still Want To Use Nails Just Go To A Bigger Size Nail. I Would Stay With A Spiral Nails If You Choose To Use Nails.

Usually, the building code requirement for 2X6 deck boards is two exterior rated screws at each floor joist. They are usually 3"-3 1/2" long, no closer than 1-1/2" from each edge of the board and should not be screwed in so deep that they break the surface ply of the
2X6. Normally, no deeper than an 1/8".

Consider a vinyl Deck Cover resurfacing solution by Plastibec
Millwork. Installs directly over any existing structurally sound PT wood deck.
Www.PlastibecMillwork.com

You are likely to experience a similar problem in the foreseeable future if the repairs are not done properly. Look for a permanent solution or call your local contractor to help you out. You will spend some money on professional but at least you will get lasting results.