You can use simple QueryOptions to do that - in this case you have to build filter which is relevant for you and you can get objects.
To get objects you can either use getObjects then you will get exact list of objects (it might be not effective from performance poinf of view. Or you can use search to get iterator to the result set (this is much more performance effective).
import sailpoint.object.QueryOptions;
import sailpoint.object.Identity;
QueryOptions qo = new QueryOptions();
Filter f1 = Filter.eq("firstname", YOUR_FIRST_NAME);
Filter f2 = Filter.eq("lastname", YOUR_LAST_NAME);
Filter f3 = Filter.eq("email", YOUR_EMAIL);
qo.add(Filter.and(f1,f2,f3));
List ids = context.getObjects(Identity.class,qo);
Iterator it = context.search(Identity.class,qo);
To get Identity knowing it’s name or id you can just use context.getObjectByName(Identity.class,NAME); or context.getObjectById(Identity.class,ID).