each sector can get 1 TRX “non hopping” (that carries the BCCH ts) + 1.3 TRX “hopping”.
In average, that’s 2.3 TRX / sector
a BTS with 3 sectors could have a config of 2/2/3, as long as the average number of TRX per BTS = 2.3 * 3 = 6.9, then you can do whatever mix you want.
1/1/5, 2/2/3, 1/2/4, etc.
of course it is much more manageable to have a regular BTS configuration.