When people talk about strength they often confuse different things like the flavor intensity, caffeine concentration, roast grade, how full bodied a coffee tastes or how much bitterness it contains.
If someone orders a coffee at a corner bakery and asks for a "strong coffee" they probably mean a high ground coffee to water ratio and a lot of dissolved caffeine. If someone complains in the same bakery about coffee that is too strong they probably mean that it is too bitter.
If a coffee professional is talking about coffee strength however, they are most likely referring to the total amount of dissolved solids (or TDS) which is measured with a refractometer. An espresso ranges somewhere in between 7% and 15% while a filter coffee is somewhere around 1,5%. That means an espresso is 93%-85% water and a filter 98,5%. The rest is what has been extracted from the coffee. Keep in mind that this is not to be confused with extraction. Higher extraction (bitter coffee) doesn't make a strong, but actually a "weak" coffee. Because you are running more water through the coffee the TDS will start to decrease at the point where the next ml of water is extracting less of the solids than the ml before. That means that in terms of flavor "strong" means often the opposite of what people would actually associate with a strong coffee. Holding the dosage constant, a strong espresso (too little water, thus something like a ristretto) would taste kind of sour, salty and unbalanced. A weak espresso (too much water) would taste bitter.
The point is, strength refers at first not to flavor but to the TDS which is determined by your coffee preparation and not by the roasting. The same coffee roasted darker is not stronger, it just received more heat.
Now, your package of coffee probably refers with it's "three out of five strength" to the roast grade (probably a medium to dark roast) which is just misleading marketing in my opinion.
Some additional reading:
Analyzing espresso recipe strength
and Espresso Recipes: Understanding Yield