I think the Model 3 will increase Volt sales even more, just as the Bolt increased Volt sales. This is natural due to cross shopping. Also, I expect the Model 3 will increase Bolt sales too. As per the Op's question, I think there will be more Volt's than Model 3's on the road for awhile. Even when Model 3 sales reach a crossover, the average Volt out there will still be putting on 8k-10k a year EV miles, not too far off from the Model 3 miles. Maybe the gen2 Volt gets 75%-85% of an average Model 3's yearly EV mileage? The biggest difference is the Model 3 can take road trips in EV mode; but the daily EV mileage should be close. We already saw the gen1 Volt with more annual EV miles than the Leaf.The Volt and Model 3 aren't direct competitors. They aren't the same type of vehicle. The Volt is a EREV or plug-in hybrid and the Model 3 is a BEV.
It's also difficult for the Model 3 to compete when it's not available for sale.