I am going through that boring stage of waiting to find out after missing last time. When I say boring, questions, questions.
I have loads of experience of litters but never seem to know. Last time I thought she was and it was just a phantom.
My question, is at 3 weeks and 5 days from last mating, should I see loads of signs or do you sometimes see none at all?
She does seem to come and sit by me a lot but then she is quite affectionate anyway. She wants to eat a lot but nothing new. Will have a scan but not going to go too early as last time had to go twice.
As she had a litter 18 months ago her nipples are quite protruded anyway.

Up to at least 5 weeks I think it"s wishful thinking

Every bitch is different, some show signs, some don't.
Do we think we see signs or is just because we know we have mated said bitch would we see these signs if we didn't know the bitch had been mated?? 
I had the bitch of my recent litter, scanned at 5 weeks from mating but by that time I was 90% sure she was in whelp.

We had a bitch a number of years ago that we swore we saw puppies moving...she never had any puppies but our eyes played tricks on us!!

I saw gloopy, stringy white/opaque discharge around Day 21 and was pretty sure from then on.

No discharges. Interesting Brainless you say possibly no signs until 5 weeks as that gives me some hope.
In earlier pregnancies I have seen discharges from later in pregnancy but not as early as 21 days.

Yes any signs before 5 - 6 weeks are due to the fact we know we have mated a bitch.

Chances are similar signs in a proven bitch (with a first litter the nipple changes are the earliest signs)would be seen after a season when not mated (the phantom pregnancy syndrome that is normal for canines), except we would not be looking for or noticing them.

With my bitches first litter she became clingy, had discharge at 3 weeks and we were sure from then that she was. And she was.
This time, my bitch had no discharge, didn't really change in temperament but did look a bit thicker in her middle at 4 weeks. Scanned at 32 days and she is- then she has expanding by the day and is enormous now at 7wks. By 35 days we wondered why we had doubted she would be lol.

A lot of this depends on how well you know your bitch!    Early days I often see the vulva hasn't quite gone back to 'normal' after a successful mating and usually by around 4 weeks going into 5, some slight thickening of the flanks.   And then there are some bitches who go 'more careful' of themselves almost from the start.   Others not so much and they still belt around with their companions until their size prevents this!

I know it can be frustrating having to wait, but by 7 weeks you will see puppies moving around.    I don't bother with scanning (which only says yes or not at the time of the scan in any case), preferring to 'assume' there is a litter on the way.   Even if I was told no on a scan, I'd still be hoping up until the due date!!

Just got back from vet & scan and yes she is in whelp at the moment
I could not use my usual lady as on holiday. It was all over in an instant but we did see 2 puppies.
Time will tell now.

The last bitch my dog mated the owners contacted me at nearly 5 weeks and said she hadn't taken, lighter in weight than before the mating and not showing any signs, very disappointed experienced breeders. She had 8 puppies last week so never say never!

I'd take no notice of numbers seen, I have yet to have more than three pups seen on a scan, and my bitches have usually carried 4 - 7 (with 11 in one case, though we lost 4 )