Are the Zwift play controllers (especially the virtual shifting) compatible with Saris H3 trainers? Thanks.
No
You can use the other functions of Zwift Play with your H3, but as Paul said you can’t use virtual shifting.
Is there any visibility into whether the Saris H3 will be compatible in the future?
Haven’t seen any info suggesting that it will. That doesn’t mean it won’t but I wouldn’t count on it. Eric asked them as part of that Zwift Insider article linked above.