Module type Async.S3
val invariant : 'a Async_kernel__.Async_invariant_intf.Async.inv -> 'b Async_kernel__.Async_invariant_intf.Async.inv -> 'c Async_kernel__.Async_invariant_intf.Async.inv -> ('a, 'b, 'c) t Async_kernel__.Async_invariant_intf.Async.inv