The printed solo-challenges don't specify starting hand limits.
they say "The players take the first turn. (...) Otherwise, normal rules apply" i can't think of any other rule applying in this case so i always thought that means 3 card starting hands
That's what I've been assuming too, but with no first-turn advantage to consider, you never know.
I have just been following the rules as written and taking three cards as well.
I don't, because it seemed to harsh. And doesn't actually make sense. But it should have been specified on the card.