So....
An easy way to explain it so anyone can understand is:
A share is valid, and paid if a block is found, for about 3 days.
The accurate way to explain it is:
The last 8,640 shares are paid when a block is found, the expected time to share is 30 seconds.
Hope this helps...
And
SPREAD = 3 # blocks
is not tell that max 3 blocks can be paid?
As far I understand, every share is valid 8640 shares of time but up to 3 blocks found in mean time.
So if we will found more than 1 block per day, and your power is not enough to get more than one share per 3 days you will not get payout from every block.