Input circuit - Current and Voltage calculation
I gather the circuit is something like this, where the \$6.5\:\text{V}\$ is a power rail and so is the \$3.3\:\text{V}\$ (but from a different power rail.)







simulate this circuit &ndash; Schematic created using CircuitLab

If so, yes -- you can combine \$R_1\$ and \$R_2\$ and the \$6.5\:\text{V}\$ rail to make up a new Thevenin source with \$V_\text{TH}\approx 3.55\:\text{V}\$. Since this is just below the zener voltage you mentioned (which suggests some kind of purposeful intention), the zener should technically be doing nothing much unless the \$6.5\:\text{V}\$ rail rises much -- when it will help hold its cathode at about \$3.6\:\text{V}\$.

Given that, and given that \$D_2\$ probably requires a few hundred millivolts to support a noticeable current (that, and given the limitations also imposed by the large valued \$R_3\$), I think any current likely to occur through \$R_3\$ and \$D_2\$ would be in the very low microamp range (or less.) Even if the \$3.3\:\text{V}\$ supply is powered down, there won't be much current. (Obviously, I'm assuming the grounds are shared.)

If you actually want to compute the current through the diode, you can use the Shockley diode equation and combine it with \$R_3\$. The resulting equation will use the LambertW equation to provide a closed solution. (See diode + resistor, for example.) Or you can use an iterative approach without it. But I'm not sure you care about any of that. (Do you?)

And what's the context for this circuit? That might go a long way in figuring out why this arrangement was created in the first place.