We have used a sample of γ+3 jets events collected by the D0 experiment with an integrated luminosity of about 1 fb -1 to determine the fraction of events with double parton scattering (f DP ) in a single pp̅ collision at √s=1.96 TeV. The DP fraction and effective cross section (σ eff ), a process-independent scale parameter related to the parton density inside the nucleon, are measured in three intervals of the second (ordered in p Τ ) jet transverse momentum p Τ jet2 within the range 15≤p Τ jet2 ≤30 GeV. In this range, f DP varies between 0.23≤f DP ≤0.47, while σ eff has the average value σ eff ave =16.4±0.3(stat)±2.3(syst) mb.