If isnull then else function with three tables

I'm currently working with three tables but I'm having difficulty including them in a if isnull formula. (The tables are my_home, other_living, and resid_care) I'm able to get any of the two tables to work when I use

if isnull({my_home}) or {my_home}= " "
then {resid_care}
else {my_home}

I tried to incorporate the third by doing:

if isnull({my_home}) or {my_home}= " "
then {resid_care}
else if isnull({resid_care}) or {resid_care}= ""
then {other_living}
else {my_home}

But it doesn't work. Please help. Thanks