First Method
Limit the first Router to 3 clients.
Meaning in your router config only allows 3 available spots in your router pool.
So for example 192.168.1.2 to 192.168.1.4
Then on the second Router, you could allow 50 for the rest of the clients and anyone that visits your location.
Second method
Give 3 of your clients, the password to one router
Give the other 4 clients password to the another router.
Third method
If your router(s) allows this, you can assign static LAN IP address to clients.
Assign each person's mac address a specific IP address. This will give you full control over heavy bandwidth users and light bandwidth users.
Then reject specific IPs from accessing either of the routers. And split up the load as you see fit.
Forth method
(I know you said you didn't want to purchase any hardware)
But I would recommending buying a stand alone Access Point that only broadcasts a Wifi signal. Make sure the AP exceeds your ISP's bandwidth limitations so you are assured that the issue isn't with your internet connect but the reliability with your access point's speed.