We have a problem with Microtic ver. 2.9.6; and wireless interface (SR5)
on (P4 on 3 GHz, mb Intel)
We make link over a distance of 45 km with the following hardware (P4,
on 3 GHz , mb Intel, Microtic ver. 2.9.6; )
and 3 wireless interface (SR5)

When we use just one wireless interface (SR5) the parametres are:
signal strength = -55/-55 , 5 Gh Turbo , nstreme enable, bandwide test - in both directions shows 37/37 mbit/s ; ccq=113/113
When we add second card in the same configuration bandwidth test in both directions shows 33/33 mbit/s
When we add third card in the same configuration the transfer bandwidth
test - in both directions shows 26/26 mbit/s

So when we have only one card everything is ok, but when we
add another card on the machine the bandwide just falls down.

When we test it on another machine the situation was the same
We test the cards on a different frequency, but nothing different happened -
bandwide is still lower when 3 cards are used

We use network bridge with every wireless on a different Lan card ,because
we need to transmit tagged VLAN s

When we use 3 wireless interfaces on one machine the CPU usage is about 60% and the traffic is about 25/10 on every line.

So do you have any ideas why the things go in this way?
Is there some kind of software or hardware limit?
Is it possible the PCI bus of the main board to cost this limitation?
We will be gratefull if you give some answers ,helping us to solve the
problem.

This is because of interference. Wireless cards should be extra shielded but best solution is to put them into different machines.

@scubydoobg

We had same problems but on 2.4 GHz frequency. There were also 3 wireless interfaces causing interference. Put them on nonoverlaping channels and it should work.
Another problem you may have is your motherboard (specifically PCI's). Try putting cards on specifically server or router intended motherboard. You will feel the difference instantlly!

See my reply to a similar issue:

http://forum.mikrotik.com/viewtopic.php?t=8139

This is the problem you are having and it can easily be repeated in a lab setup.

To solve you problem the cards need to be shielded for each other (ie. all cards need to be in different boxes or you need to design a metal shield between the cards. Technically there is no such thing as non-overlapping channels so even with 5ghz where each channel has 20mhz of spacing the issue could still be present.