Hi,
Welcome to SingAstro!!
Below are some of my view on your questions:
1) Is there alot of difference in the amount of stuff that i can see between a 3" and 5" telescope? How big a difference?
The different is quite a lot. How "deep" can you see depend mainly on Aperture... that is physics... the larger the aperture, the more light you gather... the "deeper" you can see. The quality of the optics also play a part... Although not obvious in deep sky or wide field viewing, but very obvious in planetary viewing. Also, larger aperture also allow you to see more details if the atmospheric condition allow... that's call "resolution".
2) I am told that a 3" refractor will give me pleasant views of deep sky objects like the Orion nebula. Is it true?
That's depend where you see it. A 3" refractor in Mersing or in dark sky country will show you a lots of objects beautifully, but in Singapore... err... that's another story. Orion nebula is one of the brightest nebula around... even visible to naked eye in Singapore lighted polluted sky, so a 3" scope with good weather and relatively dark environment will give you a very nice view, but I can't said that to other non star cluster deep sky object. IMHO, for someone new to observation, even a excellent 3" refractor in Singapore is a bit too small, everything will look dim and unclear.
3) In a 3" using a magnification of 90x, how big will saturn be? A dot? the size of the moon?
The angular diameter of Saturn is around 17 arcsec (1 arcsec = 1/3600 of a degree) now. So at 90x, it'll be around 25 arcmin (1 arcmin = 1/60 of a degree) which is around the same size of the Moon to the naked eye.
4) In a 3", can i see saturn in colour?
Yes.
5) In a 3", can i see nebulae in colour?
Basically no for a 3". Human eye basically don't see much colour under dim light condition. You won't see colour in most non star cluster deep sky object even you use a big telescope. Only some nebula... mainly planetary nebula, will show you some hint of colour if you use a big scope under good dark sky.
6) I am very interested in deep space objects like nebula and galaxies. What kind of telescope should i get? How much will i need to spend?
For deep sky object observing, aperture rules!! But even with a big aperture scope, it is not easy to observe deep sky object in Singapore light polluted sky... especially galaxy.
By the way, although big aperture scope will show you more, but there is a catch: big scope usually is bulky, heavy and not portable. Also, big scope usually need a suitable mount to hold it steadily and a suitable mount for a big scope is usually quite heavy too. So, for your first scope, try to get at least a 4" for Refractor and at least 5" for Newtonian/SCT/MCT, unless you really can't handle a 4" Refractor with mount or a 5" Newtonian/SCT/MCT with mount. The bottom line: Get the largest aperture scope with mount that you can easily handle and transport around (unless you observe at home)... a small scope you use often is better than a large scope collecting dust!!
Good luck in your scope hunting and have a nice day.