<...sum all the single phase loads and divide it by three to get a equivalent connected load in three phase?...>

It dpends upon the distribution of the single phase loads across the three phases.

In an ideal situation, the single phase loads will be distributed equally so as to minimise the feeder cable size and the ratings of the upstream circuit protection devices. In practice, equal distribution of single phase loads across the phases is not possible, as these are the loads more likely in some cases to be switched on and off. So some load imbalance is almost inevitable.

There may also be phase imbalance considerations as part of the utility supply organisation's tariff for the power consumed. The chances are that the red phase, or whatever the equivalent is today under BS7671(!), is more heavily loaded than the others, as ad-hoc additions to existing installations usually start at the top-left of the dis board, and the (old) red phase is usually at the top. It is therefore worth reviewing single phase load distribution from time to time to see whether economies in tariff charges can be made by redistributing single phase loads better across the (old) yellow and the (old) blue phases instead. Sometimes this can be more effective and more economical than uprating supply cables and protective devices elsewhere.