Lightroom and Photoshop can both be installed on two computers using one license for each program. Your tablet (if it is an iPad) is capable of using Lightroom Mobile. In order to use Lightroom Mobile it would be necessary for you to subscribe to the creative cloud photography plan which costs $9.99/month (US). If you don't have the creative cloud plan then you won't have access to the creative cloud service where Lightroom Mobile accesses images.
At this time there is support for android phones using Lightroom Mobile, but support for android tablets has not yet been added.