I have 10 loosely tied bags of topsoil from a nursery that have been sitting outdoors for 2-3 years. I read an answer to a similar question, but I have some follow up questions. The soil smells normal, but there is a green discoloration of the inside of some of the bags that looks like algae. In the other answer, the writer said it would be okay to use the old soil in "small" areas, and that makes me wonder whether this soil will come back to life soon enough and well enough to make vegetables and flowers grow in a 60 square foot garden this spring.
A word about topsoil as opposed to composts (either potting or composted materials) - topsoil is used where there isn't enough soil in an area already for whatever reason. It is not generally sold with added nutrients, and simply tops up the soil level where you need it to be topped up. If you're looking for something which improves the quality of soil, then composted, humus rich material is more appropriate.
Regarding the bags you have, there should be no problem with using it anywhere you need a higher soil level, but if you were using it simply to 'improve' or fertilize areas where you want to grow plants, it would be best to use humus rich,composted materials instead, or as well. How fertile or not the bagged topsoil you have is is hard to say - fertility is largely dependent on the activity of diverse life forms within the soil. Given you've got algal growth, the chances of it still being reasonably fertile are probably relatively good, because the algae wouldn't be there if the soil was completely dried out and inhospitable to life, but I'd still recommend the addition of organic composted material.
yeah, I would just mix it in to the earth and call it good... the only possible harm I could think of is if you have acid soil and the garden soil has sort of soured... like added ph 3 soil to ph 5.5 earth... but that isn't likely going to hurt anything, it will likely just make the earth better like most organic (like things from life, not like "Organic") amendments, by making the soil texture better and adding P and N.
I'm sure too that it would be fine. The algae will die once in the open and out of the bags. If there is little by the way of nutrients left in it, it will still add something to the soils structure. I'd spread it out on the top and let the birds pick away at any bugs that may be lurking. Anything that can survive will do so. It also clears away some bags you have loitering for an opportunity to be used.
Ordinary soil was created by the movement of glaciers against rocks grinding into small particles. So, the basis for soil is ground rocks, and is inorganic. When you purchase topsoil, there may be some organic matter in it (humus) but likely it's all decomposed now sitting there for three years without water, and any nutrient has either been lost, or is bound tightly to clay particles.
So, you can still use it, but it won't be as good as fresh, and you will have to add some organic material into it such as compost to increase the nutrients available to plants, and feed the soil web.