My Nikon d3100 camera keeps saying when I put my SD card in this card is not formatted. Format the card. Please help. I've tried multiple?

Cards and none will work. I have formatted them on my computer and on the camera itself and it continues to say this.

Have you checked that your cards are unlocked?

Have you consulted the manual?

All I know is:

Make sure the card you are trying to use will be compatible with the camera.
Reading the Owner's Manual should help with this.

Make sure the card isn't "locked" (check the tiny slider on one of its edges).

Always format a card in the piece of equipment that will be storing data on it ---
in this case, of course, the camera.
Formatting it in anything else is a waste of effort.

That camera accepts SD, SDHC and SDXC cards up to 128GB. If nothing works, it may be the card slot already. Shine a flashlight into the empty slot to see if there's something in there. You can clean it by holding the camera up with the card slot facing down then pump the card slot with an air blower from a camera cleaning kit (the blow brush without the brush). If it still does not work, exhale to the card slot right before inserting a working memory card. Finally, if all fails, have the camera checked at the service center.

To make sure the memory card works, format it in the computer first before trying on your camera. Years of handling various dSLRs and people handling dSLRs have taught me that there's no difference where you format the card as long as you do it correctly. When in doubt about the filesystem during formatting, choose FAT for SD cards, FAT32 for SDHC, and exFAT for SDXC.

If the card works after breathing into the slot, I suspect a loose connection inside. If you are brave enough, take out the bottom plate of your camera and find the flat ribbon cable right under the memory card slot that connects to the bottom circuit board. Push the flat cable gently towards its socket taking care not to tear anything. The retainer clip is very tiny if ever there's one. It's very easy to break too. You can force the flat cable in without taking off that clip. If the break in the connection is in the soldering, not much you can do but have it serviced.

First, did you READ your camera's Instruction Manual? It will inform you what cards are acceptable in your camera, and will also give you detailed instructions on how to format the card. That should be your first step, READING the camera's Instructions Manual.

Ensure your card meets the requirements in accordance with the camera's Instructions Manual. Make certain you unlocked the card (see the card's Instructions).

Make certain the camera's card slot contacts are clean (use a blower, holding the card slot facing the floor so whatever dust/debris is loosened falls out of the card slot).

Is the card new? Return it and get a replacement. IF the card is used, really make certain you unlocked it before trying to format it again, and consider it may be defective (yes, it happens).

Still can't format the card? Take the camera AND new card to the camera shop where you purchased it and let them try; if the counter person knows what he/she is doing, he/she will also instruct you on what to do.

I'm not certain but I DID read somewhere that some older cameras can't handle some modern cards with larger capacity. Remember, your camera is about 8 years old (and I'm not certain if this is indeed a factor, OK?). You may want to check with the camera's Instructions Manual and get the recommended card.

OR your camera may have a loose connection in the camera's card slot (I hope not)

ALWAYS format cards with whatever object (or particular camera) it will be used with… To avoid problems, mishaps and, likewise, doubt. Some photographers will tell you it doesn't matter but… Why complicate your life? Why not make matters simpler for yourself, yes or YES?

BTW, you have a nice camera; I hope you enjoy it for many years to come.

This is what I got from simply googling Nikon D3100 compatible memory cards:
The D3100 takes 3 types of memory cards…
SD - Up to 4GB worth of images and video.
SDHC - Up to 32GB worth of images and video.
SDXC - Up to 128GB (currently) worth of images and video.

If your card type is larger than the maximum capacity shown above, then you have an incompatible card. If it is equal to or less than the maximum capacity, then it's likely that there's something wrong with your camera. Yes, technically, all of cards could have the lock mechanism set to "lock," but I find that fairly unlikely that ALL of your cards would be set that way. Doesn't hurt to double check though.

NEVER, EVER, EVER format a memory card with a computer. This will install a file system onto the card with which your camera is not compatible. You must format the card with the exact camera that you'll be using. Formating a card in a D3100 and then trying to use it in a D3200 will not work. The camera which is going to be using the card MUST be the camera that formats the card.

If the cards format successfully in a computer but not in the camera, the camera is at fault. I checked the firmware updates and there's no known problem with that. So, there's a physical problem. If the card slot is clean and debris free, then the problem is probably an electrical connection problem, or possibly some electronic problem. Unless you are very mechanically competent, it is going to require a maintenance repair. Check to see the estimated cost and compare it with replacing the camera. Keh.com has some for about $200.