Reverse Engineering
From Creating Combat Encounters (DMG pp.81-84).
Encounter 1: 6 monsters totalling 600XP. This means an Encounter Multiplier of 2 so each monster is \$600/2/6=50\$XP, so CR1/4.
Encounter 2: 2 CR 2 monsters (450XP). Encounter multiplier 1.5 so \$450\times 2\times 1.5 = 1350\$ - looks correct.
Encounter 3: 7 monsters of CR3, CR1 & CR?. Encounter multiplier 2.5. CR3 - \$700\times 2.5 =1750\$, CR1 - \$200\times 2.5 = 500\$ that totals \$2250\$ so \$2150 - 2250 = -100\$XP left for the minions - so the minions are going to be on the player side? Or perhaps there's a mistake here somewhere? If these are the same type of minions from Encounter 1 then they are worth \$50\times 2.5\times 5 = 625\$. This would be a total for this encounter of \$2,875\$.
Encounter Difficulty
5 level 3 PCs have an encounter budget of:
$$\begin{array}{clc}
\text{Easy} & 75\times 5 = 375\\
\text{Medium} & 150\times 5 = 750\\
\text{Hard} & 225\times 5 = 1,125\\
\text{Deadly} & 400\times 5 = 2,000\\
\end{array}$$
Encounter 1 is Easy, Encounter 2 is Hard and Encounter 3 is Deadly. This means:
Easy. An easy encounter doesn't tax the characters'
resources or put them in serious peril. They might lose
a few hit points, but victory is pretty much guaranteed.
Hard. A hard encounter could go badly for the
adventurers. Weaker characters might get taken out
of the fight, and there's a slim chance that one or more
characters might die.
Deadly. A deadly encounter could be lethal for one or
more player characters. Survival often requires good
tactics and quick thinking, and the party risks defeat.
So, the first encounter shouldn't use up too many resources - assuming your players can acurately assess the threat and don't blow all their resources here. The second encounter will challange them and the third could be a TPK if things go wrong - I'm OK with that if you are.
Multipart Encounters
Sometimes an encounter features multiple enemies that
the party doesn't face all at once. For example,
monsters might come at the party in waves.
For such encounters, treat each discrete part or wave as
a separate encounter for the purpose of determining its
difficulty.
OK, done that.
A party can't benefit from a short rest between parts
of a multipart encounter, so they won't be able to spend
Hit Dice to regain hit points or recover any abilities that
require a short rest to regain. As a rule, if the adjusted
XP value for the monsters in a multipart encounter is
higher than one-third of the party's expected XP total
for the adventuring day (see "The Adventuring Day,"
below), the encounter is going to be tougher than the
sum of its parts.
Well 3rd level PCs have a daily budget of 1,200 each, there are 5 of them so one third is 2,000 and the total for these 3 is 4,825 - so, more than two thirds. I would try to squeese in a short rest between 2 and 3 or else the odds of a TPK go way up.
Other Concerns
Damage Immunities Bludgeoning, Piercing, And Slashing Damage From Nonmagical Weapons That Aren't Silvered
If the PCs only have normal weapons then only the Sorcerer and Cleric can damage the lycanthropes. If so, then a TPK is almost certain. Better make sure some if not all of the Rogue, Fighter and Monk have magic or silver weapons.
If the target is a humanoid, it must succeed on a DC 11/12 Constitution saving throw or be cursed with wererat/wolf lycanthropy.
Each bite, the players have a 1/3 to 1/2 chance of getting cursed - get bitten 3 times and your chance of avoiding the curse are about 12-25%. Assuming the PC wants to fight the curse (and it isn't a full moon) then this shouldn't affect this fight but it needs the 3rd level spell Remove Curse to cure which the cleric can't get until 5th level.
