SELECT users.name, users.uid, count(*) as nodes
FROM users_roles, users, node_revisions
WHERE users_roles.rid IN (3,4,5)
AND users_roles.uid=users.uid
AND node_revisions.uid=users.uid
GROUP BY users.uid;

SELECT users.name, users.uid, count(*) as comments
FROM users_roles, users, comments
WHERE users_roles.rid IN (3,4,5)
AND users_roles.uid=users.uid
AND comments.uid=users.uid
GROUP BY users.uid;

This is pretty close.. the only bug is that it won't show someone with 0 comments or 0 nodes: 
SELECT users.name, users.uid, node_table.nodes, comment_table.comments
FROM
users_roles, users,
(
 SELECT users.uid, count(*) as nodes           
 FROM users_roles, users, node_revisions
 WHERE users_roles.rid IN (3,4,5)
 AND users_roles.uid=users.uid
 AND node_revisions.uid=users.uid
 GROUP BY users.uid
) as node_table,
(
 SELECT users.uid, count(*) as comments
 FROM users_roles, users, comments
 WHERE users_roles.rid IN (3,4,5)
 AND users_roles.uid=users.uid
 AND comments.uid=users.uid
 GROUP BY users.uid
) as comment_table
WHERE users_roles.rid IN (3,4,5)
AND users.uid=comment_table.uid
AND users.uid=node_table.uid
GROUP BY users.name;