Not sure what the account is with the gallery of photos on so hard to say.
You certainly won't lose anything you currently have edited with LR3. When you first open your old LR3 catalogue with the new version you'll be asked to upgrade the catalogue so it's compatible with LR5 but you'll still have the old catalogue too should you wish to go back to LR3.
The CC licence allows you to have the software on two devices. Unless the tablet is an iPad in which case you can also LR Mobile on it as part of the Photographer's CC package too. You can't use the full versions of LR on three platforms on one CC licence though, no.
Upgrading to Lightroom 5 would involve Lightroom 5 creating a new copy of your existing Lightroom 3 catalog. Your existing catalog would remain intact and continue to be usable with Lightroom 3 if you keep it installed on the computer. A Lightroom license will entitle you to two installations for your personal use. Only one installation can be in use at any given time to be in compliance with the license. Lightroom Mobile is part of Lightroom 5. 4 and higher. You would be entitled to download the mobile application to your tablet. However, at this point, only Apple products are supported. Android support is planned for some time in the future.