good. you know, it's really a great question because it highlights a way of thinking about circuits that is useful to learn. we often look at a circuit and think in terms of voltage or in terms of current, but not always in terms of both. for the signal of the guitar, it is often natural to think in terms of voltage alone. that is helpful for thinking about R3 because you see that on one side of R3 is a stable voltage, called VB in the schem, and on the other side of R3 is the input signal coming from the TIP of the input. so R3 must be insulating the signal somehow from the voltage level VB. R3 does this by influencing the current.
without R3, the flow of current would be essentially infinite and the voltage supply would "flood" pin 3 of the LM741 with electrons that would keep the voltage level at VB.
with R3, VB pegs the "average" voltage level at VB and R3 slows down the flow of electrons enough to preserve the voltage wiggles from the input TIP. sometimes the flow is faster than others and that is what makes it possible for the voltage to remain constant on the VB side of R3 and vary on the other.
because this is a tutorial, it might be worth noting that similar things are going on with R1 and the VOLUME pot at the end of the circuit. instead of VB, the voltage level on the other side of the resistor is ground (GND). R1, which has a really high value, is the so-called "pull-down" resistor for preventing pops when the circuit is switched into the signal path. a very small current through R1 is enough to prevent a TIP voltage that could creep above ground level while the circuit is by-passed due to electrons leaking through capacitor C1.
at one extreme, the VOLUME pot allows us to "flood" the TIP of the output with electrons from ground so that the signal is wiped out and the output is quiet. at the other extreme, the VOLUME pot acts as another pull down resistor, attenuating the signal somewhat and guarding against electrons leaking through C4. this is why one does not see a pull-down resistor on the output TIP.