Add test for Lift3 for Free.

Author: Choc13

tests/FSharpPlus.Tests/Free.fs

@@ -316,3 +316,39 @@ module Fold =
             |> Identity.run
 
         areStEqual (Ok { Id = FooId "1"; Name = "test" }) response
+
+module Lift3 =
+
+    type Instruction<'next> =
+        | Read of int * (string -> 'next)
+        static member Map(i, f) =
+            match i with
+            | Read (x, next) -> Read(x, next >> f)
+
+    let read x = Read(x, id) |> Free.liftF
+
+    type ApplicativeBuilder<'a>() =
+        inherit MonadFxStrictBuilder<'a>()
+
+        member inline _.BindReturn(x, f) = map f x
+
+    let applicative<'a> = ApplicativeBuilder<'a>()
+
+    [<Test>]
+    let ``should be able to use applicative CE which requires Lift3`` () =
+        let program = 
+            applicative {
+                let! a = read 1
+                and! b = read 2
+                and! c = read 3
+                return a, b, c
+            }
+
+        let result = 
+            program 
+            |> Free.fold 
+                (function 
+                | Read (i, next) -> i |> string |> next |> result) 
+            |> Identity.run
+
+        areStEqual result ("1", "2", "3")