Allowing parties of 3 or 4 players to queue up for ranked play in was always a bit of a unique feature in Heroes of the Storm. In their most recent patch, the rules were changed so that 5-man premade parties were only queued up against other 5-man parties, while a party of any other size would enter Hero League normally. After taking some time to examine the dynamics of 3 or 4-man parties playing in Hero League, Blizzard has decided to limit HL parties to either 1 or 2 people starting with the next game update. Spyrian went into detail in an official post:

When we look at the data for parties of three and four players in Hero League, we often see large skill gaps among party members. This is fine for players in a party of four, for example, who have agreed to queue together despite any differences in skill that may exist. However, the matchmaker needs to complete the team by pulling in a fifth player who queued up alone, and did not necessarily want to play with teammates of varying skill levels.

This can also create situations in which party members will discuss strategy with each other, but don’t end up communicating the game plan to their fifth teammate. On the other end of the spectrum, solo players may disagree with the party’s decisions, or show unwillingness to help the team. This can result in a friction for both sides if communication doesn’t improve.

What’s more, parties of three and four players appear in Hero League queues far less often than solo players and parties of two. As such we feel that large parties represent a small enough portion of Ranked games that we’re comfortable removing them from Hero League in order to improve overall match quality.

Other MOBA's, such as League of Legends, have long only allowed solo or duo play in their Ranked queues. There's no question that there are players that have enjoyed the freedom they've enjoyed in HotS' Hero League, but there are several good points raised in the above post. This should overall be a positive change for the community, and might have the side bonus of giving players a more accurate read on their own individual abilities.