what is the best day to mate my 3 year old bitch

It varies greatly from bitch to bit - if you have no idea at all then your vet can do a blood test every 48 hours to tell you if she is nearly ready.

Some indicators are:

Blood changes from red to straw coloured
If you run your hand down her back and to the side of her vulva she will swing her tail in the opposite direction
Have a word with the stud dog owner - they should have experience with your particular breed which may have its own pecularities

Christine

This can be so variable, my bitches have seldom been fertile to matings earlier than 15 days, in Welsh springers that my friend owns, she always had success at 11 days, then add to that the atypical bitch that can be fertile at day 3 or day 23! Take advice from the Stud dog owner. If She is able to stay at his home then take her when they suggest and bring her home after 2 or 3 matings. If a great deal of distance is involved have blood tests done every 2 days from when she is earliest likely to be ready (as Stud owner), the Premate test will tell you when she ovulates, and she will be ready to conceive within 48 hours.