Tablespoon to teaspoon
Three teaspoons make a tablespoon almost everywhere. In the US, the UK and under the metric convention. Australia is the exception: its tablespoon is 20 ml, so it takes four teaspoons.
How to convert spoons
The ratio holds even though the absolute sizes do not. A US tablespoon is about 14.8 ml, a metric one exactly 15, and a UK one is conventionally treated as 15 too; small enough differences that no recipe notices. The Australian 20 ml tablespoon is the one that bites, because a recipe written there and followed elsewhere leaves you a third short on anything measured in tablespoons. The other habit worth having: a level spoon means levelled off with a knife, and a heaped one holds roughly twice as much, which matters for baking powder and yeast where the margin is small.
Questions
Three, in the US, UK and metric conventions. Four in Australia, where the tablespoon is 20 ml.
About 14.8 in the US, exactly 15 metric, and 20 in Australia.
Only in Australia. Everywhere else both spoons scale together, so three holds.
Recipes mean level unless they say otherwise. Levelled off with a knife. A heaped spoon is roughly double.
Sixteen in the US. A metric cup of 250 ml is a little under seventeen.