Could my daughter have chickenpox?

My son had it 3 weeks ago and I thought my 3 girls wouldn't get it as the didn't show any signs. Then 6 days ago my second youngest had a feaver and my oldest had 1 single spot. Now 3 weeks after my son had it all 3 of my girls have it at the same time. Its not nice. But it is normally in there system about3 to 5 days before spots and other symtems start to show dependind on the child but will start to scab over about 7 to 10 days after the spots appear.give plenty of fluids and try to keep her from scratching as it can scar.