Find the equivalent resistance of the circuit of 3 terminal voltage source
Your circuit is simply three terminals (lugs) on a black box with resistors inside of it. If you were handed such a box you could verify all this by using an ammeter with every unique terminal-pair permutation: ab, ac, and bc. If you read no current for any of them then you would move on to applying an ohmmeter to each unique pair, now. If you did that, you'd have three resistance measurements. Call them \$R_{AB}\$, \$R_{AC}\$ and \$R_{BC}\$.
The next step is to choose either a Y or \$\Delta\$ arrangement of just three resistors between those terminals. This is the simplest arrangement that you can reduce down towards. Personally, because it's easier to write it down, I'd select the Y form:



simulate this circuit &ndash; Schematic created using CircuitLab
You'd then just write down three equations in three unknowns:
$$\begin{align*}
R_A + R_B &amp;= R_{AB}\\
R_A + R_C &amp;= R_{AC}\\
R_B + R_C &amp;= R_{BC}
\end{align*}$$
And then solve for \$R_A\$, \$R_B\$ and \$R_C\$.
The problem is that you don't actually have an ohmmeter. You just have your brain and your original schematic to work from. Suppose you were to imagine how to go about figuring the resistance between terminals A and B? Look at the schematic and see how you'd approach that.
In the simplest case, measuring from terminal A to B, you do NOT need any specialized equations. All you need to know is how to do series and parallel combinations of resistors. When imagining the ohmmeter between A and B, terminal C is just &quot;open.&quot; So those two resistors are now in series with each other and then in parallel with another resistor, nearby. Like that, you should be able to directly work out the resistance between A and B without needing a Y-\$\Delta\$ or \$\Delta\$-Y conversion. In the other two cases, perhaps you will need one or the other. But it shouldn't be difficult to get through them.
Once you have your numbers, just solve those three equations. You can use Cramer's Rule if you want to do so by hand. Or you can use some software, I suppose. But you'll get the right answers.
Once you have the Y, you can convert that to a \$\Delta\$, if needed.