Split an argument in three

I was trying to have this task option for a task:

```
a parent SYM:VER=PATH [sym str str]
```

And I got:

```
java.lang.Thread.run Thread.java: 745
java.util.concurrent.ThreadPoolExecutor$Worker.run ThreadPoolExecutor.java: 617
java.util.concurrent.ThreadPoolExecutor.runWorker ThreadPoolExecutor.java: 1142
java.util.concurrent.FutureTask.run FutureTask.java: 266
...
clojure.core/binding-conveyor-fn/fn core.clj: 2020
boot.core/boot/fn core.clj: 1030
...
boot.core/construct-tasks core.clj: 986
...
boot.from.clojure.tools.cli/parse-opts cli.clj: 500
boot.from.clojure.tools.cli/parse-option-tokens cli.clj: 350
clojure.core/reduce core.clj: 6703
...
boot.from.clojure.tools.cli/parse-option-tokens/fn cli.clj: 347
boot.cli/assoc-fn/fn cli.clj: 79
boot.cli/parse-type cli.clj: 62
clojure.core/symbol core.clj: 579
java.lang.ClassCastException: clojure.lang.PersistentVector cannot be cast to java.lang.String
clojure.lang.ExceptionInfo: clojure.lang.PersistentVector cannot be cast to java.lang.String
```

What am I doing wrong?