* Titanium (Ti) is a transition metal known for its high resistance to corrosion. It forms a protective oxide layer that makes it relatively unreactive.

* Aluminum (Al) is also a reactive metal but is less reactive than alkali metals. It readily forms an oxide layer that protects it from further reaction.

* Platinum (Pt) is a noble metal, meaning it is very unreactive. It resists corrosion and doesn't readily form compounds.

Therefore, in order of increasing reactivity:

1. Platinum (Pt) is the least reactive.

2. Titanium (Ti) is more reactive than platinum.

3. Aluminum (Al) is the most reactive of the three.

Important Note: Reactivity is a complex concept. The specific conditions of a reaction (temperature, presence of other substances, etc.) can significantly influence how a metal behaves.