Gas Dilution

Air normally contains 78.1% Nitrogen and 21% Oxygen by volume. Argon amounts to 0.9% which can be considered as inert and can be accounted with Nitrogen. You have not mentioned whether the required final concentration is on a weight basis or volume basis. The below answer is based on a 97% volume basis.
Let Q-N2 is additional Nitrogen required.
(Q-N2 + 0.79 Q-air) / (Q-N2 + Q-air) == 0.97

Q-N2 = Q-air * (0.97 - 0.79) / (1-0.97)

Q-N2 = 3400 * 0.18 / 0.03 = 18240

If the required concentration is on mass basis, the required Nitrogen will be significantly different. It is so, let me know.

Is this a solid-gas fluidized bed, doomster? Following off of Razookm's answer, you will need to ensure that your "fluidizing" unit can handle the increased flow rate if you simply inject 18,240 Nm3/hr N2 into a 3,040 Nm3/hr air stream. There is very little chance it is sized for an ~7x increase in superficial gas velocity. You would likely end up blowing everything out the top of the fluidizer unless you can bleed the excess flow somewhere else.

Since you would be consuming 18,240 Nm3/hr of N2 for what originally is a 3,040 Nm3/hr flow, why not just go the other way and flow pure N2 with a little bit of dilution air? I calculate you would need 2,624 Nm3/hr of pure N2 and 416 Nm3/hr air to achieve 97% N2. That will let you keep your original 3,040 Nm3/hr flow rate. If you must consume all 3,040 Nm3/hr of air (blower constraints, etc) and want to keep the 3,040 Nm3/hr total flow rate to the fluidizer, you could just bleed the excess air (~2,624 Nm3/hr), which would be far less than the 18,240 Nm3/hr bleed required if injecting N2 like in your OP.

Another method would be to use an inert gas generation unit, for which 97% v/v of N2 is most likely possible. This is a pressure swing adsorption unit, there may be some TSA units that may give you higher purity N2. Talk to licensors supplying these PSA or TSA units if you dont need cryogenic quality N2 for this application. Feed air will need to be generated at oil free air compressors (to prevent contamination of the adsorption agent with oil flooded air compressor lube oil vapor carryover).

Thank you all for your quick response. To have a constant volume of fluidizing unit to the hoppers, we planned to pre-mix the gases and discharge it at the same parameters as if it was a dry air.
The set-up will be:

There will be a gas receiver before the fluidization. In the gas receiver, we planned to mix it with nitrogen to dilute the O₂, and discharge it to the hoppers at the same parameters.

Can you reduce the incoming flow rate of the air stream by either bleeding off some or via a control valve? If you can control it down to 416 Nm3/hr, you can add the balance as pure N2 flow to get your 97% and keep 3,040 Nm3/hr to the fluidizer.