The pictured Colpitts oscillator circuit will not oscillate on a plug-in protoboard without C3 and its connection to ground in place. In LTSpice the circuit oscillates with or without C3 and its connection to ground.
The LTSpice circuit oscillates at about 928 kHz and the protoboard at about 900 kHz.
I am trying to understand why C3 is necessary on the protoboard. The protoboard has a lot of stray capacitance and inductance and I have a number of long leads on the components. Perhaps that has a lot to do with it.
From a theoretical point of view, the "tops" of R1 and L1 have a AC path to ground through the DC power supply, V2. The higher impedance path through C3 seems to me to be superfluous. The protoboard is connected to an HP dual power supply.