My son and daughter LOVE parties (they're 6 and 3). Their favorite thing to do is get a bunch of flowers and pretty leaves and bring them inside and strew them about. So, if your son thinks flowers and plants are cool, you could do a lot of decorating with those! (Think back to Victorian and before times, and how decorations were mostly with seasonal colorful plants. Flowers wreaths and bouquets, strands of holly and ivy, etc).
They also LOVE balloons. So, for my husband's birthday, I printed up a bunch of pictures of balloons and hot air balloons. My kids loved coloring them (my daughter scribbled though probably 10 of them!), and I hung them all up around the house. They didn't care that they weren't floating balloons--these were colorful and fun! You could also use cardstock and just cut out a bunch of colorful balloons and hang them up
Another thing that's like balloons, but not nearly as wasteful, is balls. My kids love balls! And, they make a nice outdoor toy, too. Bubbles are a HUGE hit with kids. If you can get a bubble machine, that's awesome. When my niece had her 1st birthday, my son was 3 and spent the whole time just running through the bubbles. When I worked with preschoolers, another worker said, "balloons are like crack for three year olds." I don't think they were too far off with that statement! If you can get those giant balloon wands for them to make giant balloons, that'd be super cool!
For games, we like playing
Wildcraft (attention spans are short, so we usually just play going up to the top of the mountain, and not back down, and I often kind of rig the game so the kids hit the shortcuts to get to the top fast, ha! They often don't notice if you "accidentally" move them 4 spaces instead of 6, and they cross the log bridge to get their sooner). Wildcraft is a lot like Candy Land, but with learning about herbs to solve ailments you get along the way. My daughter is 3.5, and can already ID most of the herbs! It's not just on amazon, you can get it through
Learning Herbs and from
Mountain Rose Herbs and probably other places, too.
Candy Land is another fun one (once again, stacking the deck isn't a bad idea if the kids have short attention span, ha!)
Three year olds are pretty easily impressed with life. They don't need a ton of organized games...and often don't care about them or get much out of them. If you have a class of three year olds, you can get them to do Duck Duck Goose...kind of. It's hard...and mostly chaos. They have more fun just playing and running around. So, I wouldn't worry about doing super fun games...just make super fun places and things to play with. Tee-pees, forts, etc. You could build a few things outside quick and easy and incorporate them into any kind of treasure hunt story....though, with your child just turning 3, don't be amazed if this kind of goes over their heads.
If your kid likes trains, you could make a giant "train" by setting a bunch of chairs in a row and putting blankets on it. Or make a fort the same way. Or make an obstacle course with different things for them to run and jump and crawl through. Little kids love that!
I second the idea of a treasure hunt, too! But, of course I say that because I did
a treasure hunt for both of my kids when they were around that age. It's actually a bit above the age level for a 3 year old (I did them when I taught preschool and hid a treasure chests for both 3/4 year old class and 4/5 year old class, and the three year olds were totally lost, but they still loved finding the treasure). Both my kids were a bit lost when they had theirs, but still loved it (my son really loved when we did one for my daughter, because he was old enough to really understand it, and now they take turn hiding treasure and having the other dig it up).