When you say all of them, do you mean your three daughters and five grandchildren, eight people in all, are to receive equal shares?
Say you have £400,000 to leave. They all receive £80,000.
Or, leave it all to your daughters so they each receive £133,333. When they die, two grandchildren receive £133,333 and three grandchildren receive only £44,444.
Also, bear in mind what would happen if any of your daughters were to predecease you or divorce after you have died and find that their inheritance is taken into account in the divorce settlement.
This really depends on whether you want all of your grandchildren to benefit equally.
If that is the case, I might be inclined to set aside a specific percentage of my estate to be divided equally between all grandchildren (however many there happen to be by the time you die) and the residue divided equally between your daughters - making provision for what is to happen if any of them predecease you.