I just want to ask if 1/35 figs are proportioned with 1/48 Aircrafts and 1/32 Armors. Are they all proportioned or only 2 of them or none at all?

The scales are all independant. Something in 1/35th scale is 1/35 the size of the actual object. The same applies for 1/32nd and 1/48th. A 1/48th scale figure would be far too small for say, a 1/35th scale tank model. A 1/32nd scale figure would be far too large for a 1/48th scale airplane, etc...

32nd and 35th are close to each other but view it like thisif you were building the USS Enterprise in 35th scale it would be 377 inches long in 32nd it would be 412 that is almost a 3 foot difference for a 6 foot man the difference is 2/10ths of an inch.

So does it mean I can't make a diorama out of these 3 types of models? I really want to make a diorama which include a tank, an infantry and an aircraft which is proportion to each other.

You can, but you have to pick the same scale. 1/72 scale is a popular aircraft scale and there are numerous small tanks in this scale. Soldier figures as well, but they turn out to be only about 1" tall.

The next size scale is 1/48 scale. Another popular aircraft scale, but there isn't much decent armor in this scale or soldier figures. There used to be a nice range of Bandai 1/48 armor and soldiers, but they are no longer made. Frog/Fuman recently re-issued them, but even these are hard to find.

Then is 1/35 scale. This is the standard armor and soldier figure scale, but even the smallest jet aircraft is huge in this scale. Large scale aircraft is normally 1/32 scale, which is slightly larger than 1/35, but should suit your needs. Problem is the limitation of aircraft kits in this scale. There are several modern helicopters done in this scale (1/35) that may work for you as opposed to an airplane.

A diorama with a 1/35 scale tank and soldier with a 1/48 or 1/72 scale aircraft wouldn't be too realistic.

To follow on from Sabot's comments about availability of 1/48 scale kits, there is a nice selection of 1/48 scale kits of armour, soldiers, aircraft, etc at quarter-kit.com They would allow a nice dio of WW2 or modern era but could be expensive.
I've bought stuff from them in the past and they seem reliable, delivery took a couple of weeks.

That's what I want to prevent from happening. Why won't they make figs in a fixed scale?

Because they make 1/72 scale figures for modelers who want to do table top wargaming or to go with 1/72 scale aircraft or armor dioramas. Then they make 1/48 scale figures to go with aircraft kits in that scale. Same with 1/32, 1/35 and so on. Larger figure scales are for modelers who do not wish to build a vehicle but want to do a detailed figure.