Do you have everything else you need? Layers pellets (I presume you're getting point of lay ladies about 16-18+ weeks old) and a strong plastic or preferably galvanised bin to keep it in, drinker, feeder, bedding for inside the coop, (preferably not straw as it's not absorbent and is difficult to clean out) muck bucket, rubber gloves for poo picking, etc.
The Ronseal stuff is good and doesn't smell for so long as traditional creosote, so I should think a couple of days after the last coat has dried would be fine. I've just been using similar stuff on our summerhouse, and find it dries very fast in hot sunlight but slower in cooler shady conditions. However, the instructions on mine (solvent-based Wickes timber preservative) say you should add the next coat before the previous one is dry, which speeds things up! If yours is water-based it won't take long to dry and doesn't smell too much.
Where are you getting them from? Will they be different breeds, so you can tell them apart, and more importantly, tell which hen has laid which egg that day, as each breed lays eggs which are a different colour, size and shape from others. 3 is a good number to get as if and when one dies you wont have one lonely hen left, - but then you'll need room for two more replacements as it's very difficult to add just one hen. How big is the run? We recommend a minimum of 2 sq. metres per bird, and runs attached to a coop are usually too small for the number of birds the coop will hold, unless they're outside nearly all the time. Also, coops often claim to cater for much larger numbers of birds than is actually possible. If you'd like to send us a link to your coop and run, maybe we could advise on numbers?