V752 Cen -- A triple-lined spectroscopic contact binary with sudden and continuous period changes

V752 Cen is a triple-lined spectroscopic contact binary. Its multi-color light curves were obtained in the years 1971 and 2018, independently. Photometric analyses reveal that the two sets of light curves produce almost consistent results. It contains a W-subtype totally eclipsing binary, and its mass ratio and fill-out factor are $q = 3.35(1)$ and $f = 29(2)\,\%$. The absolute elements of its two component stars were determined to be $M_{1} = 0.39(2)M_\odot$, $M_{2} = 1.31(7)M_\odot$, $R_{1} = 0.77(1)R_\odot$, $R_{2} = 1.30(2)R_\odot$, $L_{1} = 0.75(3)L_\odot$ and $L_{2} = 2.00(7)L_\odot$. The period of V752 Cen is 0.37023198 day. The 0.37-d period remained constant from its first measurement in 1971 until the year 2000. However, it changed suddenly around the year 2000 and has been increasing continuously at a rate of $dP/dt=+5.05\times{10^{-7}}day\cdot year^{-1}$ since then, which can be explained by mass transfer from the less massive component star to the more massive one with a rate of $\frac{dM_{2}}{dt}=2.52\times{10^{-7}}M_\odot/year$. The period variation of V752 Cen over the 48 years in which the period has been monitored is really unusual, and is potentially related to effects from the possible presence of a nearby third star or of a pair of stars in a second binary.