B737 Captain reported was informed by mechanic that three MLG tires had low press and had to be replaced along with a cracked brake. Pilot noted the on the same flight number a day prior and a day after with low tire pressures.

Narrative

Around 25 minutes prior to departure; I was informed three tires on the aircraft were low pressure and apparently the pressure was low enough that all 3 had to be replaced. while replacing the tires; mechanic discovered a set of cracked brakes; also requiring replacement. Several things come to mind. Why was this not discovered earlier? 3/4 of the passengers had already been boarded. How long had the plane been flying around with low tire pressures; so low; that the tires had to be replaced? How often are the tires checked? The next day; I received a call from the captain of the inbound flight to ZZZ; same flight number xxxx; one day later. He told me 25 minutes prior to departure a mechanic told him a tire had low pressure and had to be replaced. The gate agent had worked my flight the day prior and mentioned to captain exact same thing happened day prior. We discussed this and found it was the same mechanic. This mechanic had quite an attitude; noted by me and the other captain.Something is very odd here. Why are these being discovered so late in the process of departure? How can this many tires be so low on pressure they require replacement? Either the tires are defective; that's bad. Or they need air far more often than is being currently done; this needs to be fixed if the case; or there is something else amiss. That's two very delayed flights with the same issue; something I'd say looks like a trend.