Sorry to seem a little hard-nosed, but you really need to give us (collectively) more information.
When you say 802.3, that means Ethernet (CSMA/CD) at 10 megabit on a coaxial network – that being the original 802.3. If you mean something else, then you need to specify exactly what you do mean.
If that is really the case though, how do you know? What indication is telling you about the collisions? If it’s a coax network, have you tried segmenting it by inserting terminators to isolate the problem – strictly as a temporary troubleshooting measure? I could go further, but I’ll wait until you confirm the precise nature of your layout.
Since coax is getting rare these days, then let’s lay out the other possibilities.
If it’s switched ethernet (of any speed), do you have a managed or unmanaged switch? A managed switch will allow you to identify the precise port from which the collisions are originating. As for how-to, that depends on the brand and model of the switch. Not even going to try without that info.
Lacking that (a managed switch), if you have status lights, which are indicating the problem, then unplug one station at a time until the status lights stop indicating collisions. Then go investigate what’s on the other end of that wire.
Also – Please feed back the answers to the group as a whole. One of the key reasons that we’re part of this group is so that we can learn from the experiences of others. Questions that offer little in the way of information are likely to get ignored.