Water coming in through backdoor threshold
If possible, unscrew and remove the threshold.  Clean it (if your threshold is bent or dented, it will need to be replaced) and the area around and underneath it thoroughly with soap and a bit of water.  Let dry thoroughly, perhaps overnight.  Buy a tube of clear silicone caulk.  One tube is enough. 
 Rescrew down the threshold.  If the screws do not tighten down, use thicker screws.  Make sure the threshold doesn't move.  If you cannot remove the threshold, clean it where it is.  Now take the tube of caulk and run a 1/8" bead  around the edge of the threshold inside and out and completely fill the spaces between the threshold and door jambs.  Use as much as necessary to fill all openings around the threshold.  Wet your finger and smooth out the caulk.  Any excess caulk on the threshold can be removed carefully with a wet rag now before it begins to dry, or leave it.  There should be absolutely no openings under or by the sides of the threshold.  Let dry for a full day.  Now do your hose test.  If water is still getting in check the caulked areas.  If all is good, then you will need to purchase a drip-cap door-sweep and attach it to the bottom of your door according to the package's instructions.  There are various configurations, including attaching to outer face of door, or attaching under door and also to the outer face.  Test again.  If there is still water, then there is an opening or issue which I haven't addressed and you'll need to ask for help again.  Good luck!