Hi,
Some calculations:
1.8W @ 24V means 350 Ohms
Time is 3s, but for a voltage drop of about 50% you need about 0.5 tau (raw estimation).
So if you need 3s then tau needs to be about 6s.
Tau = R x C --> C = tau / R = 6s / 350 Ohms = 0.017 F = 17000 uF
I recommend to build a 24V capacitor buffer with this 17000uF and a decouple diode just to supply the relay.
Don't connect the capacitor in parallel to the coil.
In doubt post your actual circuit. (Power suppl to coil, with switches)
Klaus