The trio each finished regulation play at 7-under-par 123, and Jamieson went on to card pars on both extra holes to earn the European Tour victory.
The Scotsman proved steady in overtime after firing an 8-under 57 in Sunday's final round. The tournament was shortened to 36 holes after heavy rain pushed the start of the event from Thursday to Saturday.
De la Riva (61) fell out of contention with a bogey on the first extra, while Webster (60) did so on the second hole, handing Jamieson the title.
Sharing fourth place at minus-6 were first-round leaders Tim Clark and Morten Orum Madsen, each of whom shot 1-under 64, as well as Maximilian Kieffer (62) and Matthew Nixon (61).