We report the discovery of three transiting low-mass companions to aged stars: a brown dwarf (TOI-2336b) and two objects near the hydrogen burning mass limit (TOI-1608b and TOI-2521b). These three systems were first identified using data from the Transiting Exoplanet Survey Satellite (TESS). TOI-2336b has a radius of 1.05+/-0.04 RJ, a mass of 69.9+/-2.3 MJ and an orbital period of 7.71 days. TOI-1608b has a radius of 1.21+/-0.06 RJ, a mass of 90.7+/-3.7 MJ and an orbital period of 2.47 days. TOI-2521b has a radius of 1.01+/-0.04 RJ, a mass of 77.5+/-3.3 MJ and an orbital period of 5.56 days. We found all these low-mass companions are inflated. We fitted a relation between radius, mass and incident flux using the sample of known transiting brown dwarfs and low-mass M dwarfs. We found a positive correlation between the flux and the radius for brown dwarfs and for low-mass stars that is weaker than the correlation observed for giant planets.