I had the dubious pleasure of conducting a 3-eng ferry flight on a Vanguard (VC9) in the early `70s.
There were quite a number of different performance considerations eg. 2-eng inop. climb + VMCA2
The procedure was, as others have stated, basically full power against the brakes on the SYMMETRICAL ENGINES. Then a lower power on the "third" engine - followed by brakes off and the power from the "third" engine fed in progressively as speed is gained. Obviously with a much higher Vmcg used. I seem to remember ( I think ) that there was no V1 as such.
Much later in life I did undertake 3-eng ferry simulator training for the B747 but never did one in anger.
With the advent of wide body freighter aircraft which could carry spare engines the procedure would appear to be much less common.
Of historical interest: many 3 and 4 engined aircraft were fitted with an underwing "hardpoint" for carrying a spare engine. I have even seen a VC10 with a 5th engine podded under its wing - most strange sight!!