Close family, cake and prezzies, home, wine for the grown-ups, a couple of hours. 50 quid max You are about to start years of children's birthday party hell when he gets to 'real school', don't anticipate it when he is too young to understand or remember. At 3yo he'd far rather be the centre of attention of his adoring GPs/Aunts/Uncle/sibs/parents friends than overwhelmed by a lot of screamy, noisy, over-exited 3 year olds. Party-pooper? Moi?
Amazon is great for little party bag fillers like mini crayons & colouring books bubbles tattoos stickers also a little bag of haribo a mini choc bar& slice of birthday cake. I'd opt just for general fancy dress rather than a theme. I think a bouncy castle some balloons to run around with and some party songs will be enough for them. Tip - make sure there's a bottle of wine chilling in the fridge for you when you get home.
Don't underestimate the work involved,and if possible rope a few extra adults in to help you.
Make all the food and drink self-service, eg a couple of iceboxes with cans/bottles for the adults and jugs of juice with cups stacked rather than making tea/coffee or pouring drinks.
Remember to leave yourself enough clearing up time after the party before the end of the hall hire period.
Bouncy castle, balloons, bubble machine all sound good. Also bring a selection of your own and friends ride on toys, and things like hoops, balls etc. Basically, the types of things you get in playgroups (the big items - eg not colouring/play dough, or trainsets/cars). They'll have a great time running around like they do at soft play / playgroup.
Party bags are nice, since they give a structure to the goodbyes, and means you speak to each guest as they leave. They don't need much in them: eg a fancy balloon, a little thing of bubbles and a chocolate.