We present new radial velocities and photometric observations of three double-lined eclipsing binaries LL Aqr, MP Del and NSV 20913. This paper is the third in the planned series of investigations. Standard wide-band indices for the systems and comparison stars were determined accurately. Using the times of mid-eclipses, new ephemerides are calculated for the three systems. These systems consist of two well-detached components in eccentric orbits. The effective temperatures of the hotter components and reddening of the systems have been estimated from Johnson wide-band UBV photometric calibrations. New UBV photometric data and radial velocities were analysed simultaneously for the systems' parameters using Wilson Devinney ( WD) code. The masses have been found to be 1.223 +/- 0.058, 1.056 +/- 0.051; 1.559 +/- 0.085, 1.248 +/- 0.074; 1.76 +/- 0.13 and 1.28 +/- 0.11 M(circle dot) for the primary and secondary stars of LL Aqr, MP Del and NSV20913, respectively. The radii of the components are derived in the case of LL Aqr: 1.325 +/- 0.020, 1.005 +/- 0.016; in those of MP Del: 2.428 +/- 0.046, 1.575 +/- 0.030; and NSV20913: 3.243 +/- 0.081, 1.291 +/- 0.033 R(circle dot). The masses, radii and effective temperatures of the component stars have been determined with an accuracy of 5-8, 2-3 and 1-3 per cent, respectively. We have calculated the distances to the systems of LL Aqr, MP Del and NSV 20913 as 123 +/- 9, 149 +/- 7 and 246 +/- 9 pc, respectively, using bolometric magnitudes and empirical bolometric corrections for the component stars. The stellar model prediction match the measured properties of LL Aqr for an age of about 1.78 Gyr, indicating the components near the half of their core-hydrogen-burning lifetimes. However, the components of MP Del have an age of 1.59 Gyr, the primary component being turn off from the main sequence. The age of the NSV20913's components is about 1 Gyr, indicating the primary component seems to have evolved just slightly off the main sequence but the less massive secondary component is still close to the zero- age main sequence. A subsolar fractional metal abundance, Z approximate to 0.008, provides a better match to the absolute parameters of all three systems.