In Lake Placid we are introduced to Delores Bickerman. In Lake Placid 2 we are introduced to Sadie Bickerman, Delores' sister, living in Delores' old house. In Lake Placid 3 we are introduced to Sadie Bickerman's nephew Nathan Bickerman. Lake Placid 3 tells us that Nathan spent time with his aunt Sadie in her cabin on the lake as a child; even though the second movie made it clear that Sadie took over her sister's home when she died and had never lived there before.
In the scene where Vica returns to the house and has blood all over her right arm, when the camera switches the blood goes from her right to her left then back and forth throughout multiple switches.
When April goes skinny dipping, you can see that she has underwear on several times after she's entered the water completely naked.
When Reba and hunting party are in the boat, Reba slams the outboard controls into reverse, but the boat lunges forward knocking down the hunting party.
At the lake, Charlie is wearing a t-shirt that says "Welcome to Summer Delight." As Ellie and Tara leave to put on their bikinis, he turns to the camera and the lettering on his T-shirt is suddenly reversed, showing that the image is flipped horizontally. This can also be noted in the position of Tara's wristband.
In multiple scenes where mobile phones are used, you can clearly see that all of them are dummies/fake phones since the displays are not lit and there are paper behind the display glasses instead of real LCD:s