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I'm wondering whether you might just be pushing too much traffic too quickly at your 3725. The 3725 is only rated at 100 to 120 Kpps. Two things you might try, set the links to 10 Mbps or, if supported and not enabled, enable router ingress flow-control. Either might help with the 3725's input errors.
For the 3750 switch's egress drops, here too using ingress flow control might throttle the sender.
If you have time-sensitive traffic, you might not want to use flow control. In that case, either you need to accept the drops as normal for oversubscription and/or attempt to identify the high rate sources and reduce their bandwith to the network.