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Resistors in series carry the same current, so their resistances add: Rtotal = R1 + R2 + R3. The current is set by Ohm's law on the total, and each resistor drops a voltage in proportion to its own resistance, which is exactly how a voltage divider works.
Series resistance
Rtotal = R1 + R2 + R3; I = V / Rtotal
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.
The same current flows through each, so the voltage drops add, and by Ohm's law that means the resistances add.
The largest one. Power in each is I squared times R, and since current is common, dissipation is proportional to resistance.