That is going to depend on which 3.5 speaker really, not all speakers measure what they are named as. What brand do you have in mind? Many people have put in Infinity 3.5s with no issue. I had to make baffles/trim rings for my Hybrid Audio 3.5s and others have done the same for other brands.
Other fitment issues can be that the magnet is too large to fit in the area under the dash (very unlikely) or that the speaker won't fit under the grill once it is re-installed (either the speakers projects too far from the dash or the mounting ring causes it to). Its very easy to remove the grilles and stock speakers, why don't you try sizing a few speakers in there and see which ones will work best. When properly done that dash location can produce some great results.