3 Sims will work in any unlocked device.
3 Support are feckless.
Please forgive the question but the sim cards you're using are Standard size? The SG Ace takes a Standard sized sim card. Micro sims will fit into a standard sized slot but they will not work in the slot.
If the Sims are the correct size and are working in an alternative device then the problem points toward the SG Ace obviously.
First of all, have you tried any other sim cards in the device? Do any work?
Second option is to turn off the device, pull off the back cover and remove the battery. Leave the battery out for about 2 minutes, reinsert, fix back panel back on and reboot the device and retry the Sim cards.
Next, have you checked the APN (Access Point Names), settings are correct? The APN settings can be checked/changed by: tapping the menu button, (the menu button is located to the left of the physical Home button, the Home button is located at the bottom of the screen on the front of the device). Tapping the Menu key will bring up the 6 panel option at the bottom of the screen. Next click Settings, then Wireless and Network, scroll to bottom of screen and tap on Mobile Networks, then tap on Access Point Names, make sure 3 is selected if not, select it.
If that doesn't resolve the issue, there may be a problem with the device firmware interfering with activation of the Sims to the network. Have you tried updating the device firmware at all? You won't lose anything by updating but do back-up/sync your contacts to Google first just to be sure. To check for and/or update the device firmware, turn on the phone, make sure you have Wifi switched on and that the device is communicating with the router (there should be a Wifi icon at the top of the screen). Now tap the Menu button. Tap on Settings. Now scroll all the way to the bottom of the screen and tap on About Phone. The very first option in the next screen will be Software Update. Tap that and then tap Update - Check for updates to see if there's an update. If there is, download and apply it, retry the Sims following the update.
If no resolution at that juncture I'm somewhat at a loss as to what may be wrong however we may need to check off the APN settings, they're not always correctly set by the device. I don't get on here much these days but I'll try and look back to see how you get on.
