transposition

[untold kestrel]

And how to make r3 the subject also

[azure sierra]

Take the reciprocal

Subtract 1/R2 subtract 1/R1

Take the reciprocal

Apply same concept for R1 and R2

[untold kestrel]

I got shown but I still find this that hard to understand / comprehend so I need to work on this

[warm carbon]

you can "unwrap" these fractions by multiplying both sides by the denominator:

,,A=\frac BC\implies AC=B

[obsidian creek]

mtt07734

[warm carbon]

here your denominator is a bit larger but it still works

this is what it should look like after multiplying both sides by the denominator:
||RT (1/R1 + 1/R2 + 1/R3) = 1||

[warm carbon]

afterwards, multiply both sides by R2
if you closely read what Ive said, you can figure out that this is to make sure ||R2 is no longer in the denominator||