You can configure routed ports in the 3750.
Example,
interface fx/x
description 2821
no switchport
ip address 192.168.28.21 255.255.255.252
interface fx/x
description 2811
no switchport
ip address 192.168.28.11 255.255.255.252
then use a dynamic routing protocol or stating routing for balancing both links.
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Keep in mind, you are affecting outbound traffic from the 3750 toward the MPLS routers, you aren't affecting inbound traffic on the MPLS routers' WAN links.
That would be a separate configuration and/or requirements.