protocol

player says: ["action", { type: "draw" }] // draw a card

server responds: ["result", { type: "draw", card: x, red_threes: [y,z] }]
You drew card x and these red threes.  The card is added to the hand, the red threes are added to the pile of red threes.

server broadcasts to all other players: ["result", { type: "draw", player: x, red_threes: [y,z] }]
Player x drew a card and these red threes.  One card is added to the hand, the red threes are added to the pile of red threes.


player says: ["action", { type: "grab", cards_from_hand: [...] }] // grab the pile
The server can figure out which set on the table (if any) will be used from looking at the top card on the pile.

server responds: ["result", { type: "grab", cards: [...] }]
Take the top card of the pile and the cards_from_hand and add them to the corresponding set.  Then add the rest of the pile to the hand.

server broadcasts to all other players: ["result", { type: "grab", player: x, number_of_cards: n, cards_from_hand: [...] }]
Player x took the pile using these cards.  Update the player's sets and hand size accordingly.