Shulchan Arukh, Orach Chayyim 135:8 says that during Torah reading, if there is a Cohen present but no Levi, the Cohen gets the first two aliyot.
Let's say that on a Monday or Thursday, when there are only 3 aliyot, two Yisra'elim need to be honored - e.g. a Bar Mitzvah boy and an aufruf. (See explanation, below.) Can the Cohen forgo receiving the Levi aliyah and can that be given to a Yisra'el?
(I have not used chatan or sandak since there is an opinion mentioned in OC stating that for these two people, you may be able to add a 4th aliyah even on a weekday.)
In my shul, congregants get easily offended if you don't give them an aliyah on a special occasion. In this scenario, there is a Bar Mitzvah and aufruf on a Monday. The Cohen is the Bar Mitzvah boy's grandpa. If we don't give aliyot to each of the other two, they would be so offended, that we could lose these regular "minyanaires". It is difficult to get subs. Thus, they need to get these aliyot.