A 3 litre is fine, unless you want to bulk cook things like beans and pulses or make stock or preserves in it, it will feel a bit small then. If you have a freezer, I'd recommend doing this, as you will save more time/fuel/money. The other thing to consider is the height - a pressure cooker is very versatile if tall enough; you can pot roast chicken in them, there are lots of dishes you can make on a trivet, including steamed puddings, sweet and savoury custards, cheesecakes, pot roastall kinds of things. If you are likely to want to use it for more than stews and casseroles, I would go for a bigger model - a 4.5 or 6 is good.
You can get electric pressure cookers which plug in, but I don't find them as efficient. There are no issues with those which sit on the hob - today's pressure cookers have several safety features which means you shouldn't worry about using them.