Running out of hot water after 2-3 showers

[Matt B.]

Title says it all... this is in my personal residence but I’ve found BP to be a more credible source for answers than searching the web. After 2 or 3 showers the water gets cool/lukewarm, not freezing and will stay that way, can usually get away with a shower in the morning and one at night without problems but if it’s the kids in the tub and a shower that pretty much uses it up. Typical usage, nothing too long, any help? Also the water never gets really hot even though the unit is turned up. Thanks in advance.