I have a subpanel in a detached garage that has 3 wires run from the main panel. To the main there is 400 amps run from the electric provider with 3 wires, and it is split to have 200 amp service to the garage with 3 wires. I know code now says it should be 4 wires, but what can I do to add safety to this service short of digging up concrete and dirt and adding that 4th wire?
When the building was built code was probably 3 wire ( this was the standard for decades). What you have was quite common until the1999 NEC code change. A 200 amp feed would normally be in conduit and if metallic that could be used to meet today’s code. If it is a direct buried feeder and you feel the need to update it a separate ground wire is now allowed, in most cases a panel that was 3 wires will have the neutrals and grounds connected or even on the same buss this will need to be changed to isolate the neutral from ground. Also every receptacle will require GFCI protection to bring it up to current code but this is not required as your existing configuration is grandfathered. If you really think it is unsafe add the GFCI protection.
As Ed Beal says, check if it's in conduit -- that'd be an easy way to add that fourth wire, which is the best solution.
However, in case it's not in conduit, I want to also answer your direct question of if there's a way to add safety to a grandfathered 3-wire feed without adding the fourth wire, because there is: make sure all your feeder connections are torqued to spec.
The main purpose of the fourth wire is to protect against some very nasty failure modes if one of the three wires (especially the neutral) loses its connection for any reason. Since with a three wire feed, your neutral and grounds are connected at the panel, if the incoming neutral connection is broken, suddenly all your grounded equipment housings have live voltage on them. So one way to add safety to a three wire feed is to take extra care to make sure it's unlikely that the feeder wires will ever become disconnected.
Another change to the code in recent years, which didn't apply when your three wire feeder was installed, is that all connections have to be torqued to spec with a torque wrench. For many years, "good and tight" was considered fine, but it turns out that if you want the connection to last through many years of thermal expansion and contraction, you really need to be pretty precise with the torque, since either too loose or too tight can cause the connection to fail earlier than it should. This is especially important with aluminum wiring, which most large feeders are. Plus some studies have showed that us humans, even professional electricians, are not actually very good at estimating the right level of torque by hand. Therefore, modern code requires everything be correctly tightened with a torque wrench.
So my recommendation would be to get a torque wrench, and (after turning off the power), make sure all your feeder connections are torqued to spec, and correct them if not. There's a good chance the ideal torque values will be in the equipment's instructions, but if not, google it or ask here and we can help you figure out the right values.