I have Lightroom 3 and am considering upgrading to Lightroom 5. I love Lightroom 3, and would like to keep it installed as backup / alternative. I inquired with purchase chat and was told that if I upgraded to 5 that 3 would be deactivated. Is this correct? They seemed positive that if I upgraded to 5 my previous version would be deactivated. Maybe I won't like 5 as much. I love 3, and don't know that I 'need' 5, but I'm setting up a new machine so am upgrading or adding a lot of software right now as a part of that process.
If I'm going to lose 3, then I probably won't upgrade to 5. Why tempt fate and give up something that works well and that I know pretty well.
I cannot speak to any deactivating. However, the databases for v3 and v5 are not compatible. If you update your v3 database to a v5 database, LR3 will not be able to read it. No new databases created by LR5 will be able to be read by LR3.
In all honesty, I cannot think of a reason to stay with LR3 with v5.7 being available.
I know that v4 and v5 work fine side by side, so I don't see why v3 and v5 would behave any differently. Make a copy of your old catalog and update the copy to v5.