Supermassive black holes ( SMBHs ) are found in most galactic nuclei .
A significant fraction of these nuclei also contain a nuclear stellar cluster ( NSC ) surrounding the SMBH .
In this paper , we consider the idea that the NSC forms first , from the merger of several stellar clusters that may contain intermediate-mass black holes ( IMBHs ) .
These IMBHs can subsequently grow in the NSC and form an SMBH .
We carry out N -body simulations of the simultaneous merger of three stellar clusters to form an NSC .
We investigate the outcome of the simulated runs containing zero , one , two and three IMBHs .
We find that IMBHs can efficiently sink to the centre of the merged cluster .
If multiple merging clusters contain an IMBH , we find that an IMBH binary is likely to form and subsequently merge by gravitational wave emission .
We show that these mergers are catalyzed by dynamical interactions with surrounding stars , which systematically harden the binary and increase its orbital eccentricity .
The seed SMBH will be ejected from the NSC by the recoil kick produced when two IMBHs merge , if their mass ratio q \gtrsim 0.15 .
We find that the other channel , namely , dynamical interactions between an IMBH binary and a third single IMBH , is rarely effective in removing the seed .
If the seed is ejected then no SMBH will form in the NSC .
This is a natural pathway to explain those galactic nuclei that contain an NSC but apparently lack an SMBH , such as M33 .
However , if an IMBH is retained then it can seed the growth of an SMBH through gas accretion and tidal disruption of stars .