I have a Bosch PKS 16 which has a 65mm diameter saw blade. For fast long straight cuts using a fence it's excellent. For convex curves its not too bad if you take your time and don't try to get too close to the line. For concave curves I use a bandsaw if possible or a jigsaw. The Bosch is not good for concave curves but I guess that not too many circular saws are. 

Certain power tools can help you make huge mistakes really fast.  I cut out every plank with a Japanese pull-saw and was able to have control and follow the line with very little cleanup.  I even had a piece of masonite sandwiched between the plywood for making templates.  Yes, one of my arms is slightly larger than the other, but it was a very Zen experience to make most of the boat with hand tools. 

   i'm building a teardrop camper and i discovered the Dremel UltraSaw.  it is lightweight, easy to use with one hand, the blade is forward of the centerline, and you can see the blade while cutting.  i was very successful tilting the saw on the front of the shoe plate, then carefully plunging it right on the cut line.  the supplied blade for plywood cuts actually has a "ragged" edge rather than teeth.  while it makes lots of superfine sawdust, the cut is very smooth.

   I own or have owned the whole DeWalt toy kit and the Dremel one - you know, circular saw, recipro saw, flashlight and drill?

I was hesitant to buy the Dremel tool, since I had used DeWalt an awful lot. But, I was expensing some of the stuff through work, and they said "No" to paying an extra $100 or so bucks for the same tools in the neat yellow color.

I've never been sorry. The Dremel batteries last a lot longer than my DeWalt bateries do - though DeWalt DOES offer a 15 minute recharger, and to my knowledge, Dremel does not - and the tools are more solid-feeling than the DeWalt stuff. They also have a rather more pleasing feel to them, as they're done in some sort of grippy, shock-resistant plastic.

Now, I'm not DeWalt bashing, here. They make a good tool....but IMO where price vs. performance comes in, Dremel wins by a wide margin.

   Spend the $$ and get yourself a decent 7 1/4" saw that'll last you a lifetime. It'll get you through your boat build, kids' forts and anything else you need to build. I don't have any experience with a 3" saw, but like all saws, it's all in the blade you put on it. Something like this:

https://www.amazon.com/Makita-5007MG-Magnesium-4-Inch-Circular/dp/B000N5SVEY

Captain Skully said, "   Certain power tools can help you make huge mistakes really fast. "

.

I like this statement. Its true. But the opposite is also true. Both rely on the skill and judgement of the craftsman.

   I’d also like to recommend the venerable Makita SHO2Z cordless (!) 3-3/8” circular saw. They’ve been making this for over 30 years, and now offer it with lithium ion batteries and a carbide tipped blade.   In thin marine ply, it cuts smoothly and controllably.  I’m currently starting to build Mike Waters’ W17 trimaran, and I bought the saw based on his unqualified recommendation.  Sure, it won’t cut through a 2x4, but for it’s intended purpose, it shines. 

   I’d also like to recommend the venerable Makita SHO2Z cordless (!) 3-3/8” circular saw. They’ve been making this for over 30 years, and now offer it with lithium ion batteries and a carbide tipped blade.   In thin marine ply, it cuts smoothly and controllably.  I’m currently starting to build Mike Waters’ W17 trimaran, and I bought the saw based on his unqualified recommendation.  Sure, it won’t cut through a 2x4, but for it’s intended purpose, it shines. 

  I've been using cirular saws for 40 plus years. Most people are right handed. They sell those  hideous saws with the blade on the right side which you have to keep looking over your arm to see your cut. Good skilsaw types have blade on left, much easier.  one of the best saws out now is the Milwalkee 6-1/4 cordless saws. Great saw, battery lasts a long time.