If you are running a TPMS (which you, of course, should be), you should have received plenty advance notice of needing to add 3 to 5 psi. This slight loss of pressure is due to normal air loss and pressure change due to change in ambient temperature. You can easily top off your tires at your next fuel stop.
If you don't have TPMS and discover you have been driving on a tire that needs more than 20% of it's required inflation, you should be calling road service and have the tire changed, as there is a good chance you may have done permanent internal structural damage. I consider this operation on the under-inflated tire made the tire unsafe to re-inflate until the tire has had a complete internal and external inspection by a trained tire service person, not just the guy that mounts tires who probably has not received the training.
AFTER the inspection, the tire should only be inflated in an approved safety cage as doing otherwise can lead to serious personal injury.
Regarding how to find trained, certified tire inspectors... Use THIS link from Tire Industry Association.
There is a directory that you can search by zip code. Those listed are TIA Members, and those with the Certified Patch next to them have been TIA Certified.
I will suggest that folks with 19.5 or larger tires or with Load Range E, F, G or higher or with any steel body ply of any Load Range go to Certified, Commercial inspection
People with Passenger, LT or ST type tires of lower load range, can use the "Automotive" link but a certified Commercial person should be able to inspect smaller tires too.
Remember as Sgt. Esterhaus said 'Let's be careful out there'