I would like to create a map with the help of tectonic plates. I didn't really care before. Can tectonic plates have all possible sizes and shapes? As an extreme example, could a continent the size of Australia have three small tectonic plates colliding with each other?
Tectonic plates come in various sizes.
Among them there are also the so called microplates,
These plates are often grouped with an adjacent major plate on a major plate map. For purposes of this list, a microplate is any plate with an area less than 1 million km2. Some models identify more minor plates within current orogens (events that lead to a large structural deformation of the Earth's lithosphere) like the Apulian, Explorer, Gorda, and Philippine Mobile Belt plates.
On the other hand, the Pacific plate cover up 103,300,000 square kilometers.
So, answering your question, it is possible to have plates of various sizes.
To answer your specific question, technically no... because it's three continents colloding at some point in the center of a land mass the size of Australia. The Land mass of Australia is in the dead center of the Austrailian Plate, and is one of the most geologically inert locations in the entire world. A continent can sit on multiple plates (For Example, most of Main Land North America is on techtonic plate from the Carribbean Plate, even though the Islands are all considered part of the Continent of North America. Same is true with anything west of the San Andreas Fault in California.
Tectonic Plates can be made up of bodies of Continental and Oceanic Crust, with the former being thicker than the latter by about 3 km. The dividing lines are given three different classifications based on interactions.
Divides are crated when two plates move away from each other. The currently most famous divides are the Atlantic Rift, which aside from Iceland, is largely submerged and is the source of almost all images of lava under water. The other one is the African Rift Valley (Birth Place of Homo Sapiens), which is a division forming in the African Continent that is still largely above sea level.
The next important divide behavior is subduction. Unlike Divides, Subduction fault are normally the result of continental crust meeting Oceanic Crust on the opposite side. Because Continental Crust is thicker as the plates collide, the Continental Crust forces the Oceanic Crust underneath of it. The most famous example is the Pacific Ring of Fire, which is a line of volcanos on the coastal nations of the Pacific Ocean that is the result of the fact that Pacific Plate grows smaller each year, pushing Asia and the Americas closer together. Volcanoes typically form several miles inland where the subducting ocean crust forces the mantle (the layer under the plates) upwards into the crust.
The Final meeting place are transform faults. These form when two crusts with similar thickness meet and exhibit different behaviors. Since neither is denser than the other, noting will be subducted but the plates will interact by trying to move around the other one. For example, the San Andreas fault is sliding against each other, creating the well know fact that Los Angeles and San Francisco are moving closer together.
Meanwhile, India is currently slamming into Asia, causing both sides to buckle upwards, creating the Himalaya mountain range.
For your continent, to have three plates meet, it would likely create large earthquake prone mountains, but if you're seeking volcanoes, there could be another way.
Hawaii, as you may notice, is right smack dab in the middle of the Pacific Plate. As I said earlier, this has made Australia famously inert... but Hawaii is home to one of the most active volcanos in the world, which is in a near constant erputpion. The reason for this is Hawaii exists over what's call a "Hot Spot" which is a particularly active area in the earths mantle for no known reason... Hawaii (specifically the big Island but all of the chain exist because area) sits on one such hot spot and all the Islands in the Chain were formed by it, and the Islands formed as the Pacific plate was dragged over the eruption area.
Size yes, within reason. You can't have a plate that takes up half a planet and below a certain size its not really a plate because it does not reach the mantle, but you have a huge range in there.
Shape no, plate boundaries tend towards linked triple junctions (120 degree angles) at spreading centers and thus continental margins and wide shallow curves at ocean to ocean subductions zones, this is purely due to the interaction of forces and the strength and thickness of the rock. The most glaring errors I see in fake tectonic maps is long smooth lines for continental boundaries and coastlines or 4 or more plates meeting at one point.
Yes you can have three plates meeting in a australian size continent, for a given value of "in", at least one of them would have to be oceanic likely two of them, continent to continent collisions are exceedingly rare. Three plates meeting is actually quite common. Any point on a plate boundaries are almost exclusively 2-3 plates, no more, no less. But of course a plate has a boundary all the way around it so it may have more plates meeting it. The indian plate is in contact with four other plates, and the arabian plate 3.
This video may help you, it is about building fictional tectonics.