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Your routers would have more QoS features than your L3 switches, and they can pretty easily meet your requirements for outbound traffic. However, even with their QoS features, managing inbound bandwidth doesn't work too well. What you need to do is manage the other side's outbound (i.e. your inbound), but at you mention Internet access, this is unlikely to be possible.
There are a couple of inbound techniques, you can try, but again, they often don't work as well as desired. You might police inbound traffic or you might shape outbound TCP ACKs.
For your outbound, you'll want a CBWFQ parent policy that shapes for the total uplink bandwidth (750 Kbps?), and they you can use a child policy to prioritize or set aside minimum bandwidths for your outbound traffic.
e.g.:
policy-map parent
class class-default
shape average 750000
service-policy child
policy-map child
class 1st-device
bandwidth percent 67
class wireless
bandwidth remaining percent 99
class class-default
bandwidth remaining percent 1
