I have 3 tables that I need to join.

Descriptions of tables
player - Contains player's name and id
game_player - Contains the player and what game they played in and for how long.
weapon_t - Contains a table of what weapon what player used to kill anyone per game.

DESCRIBE TABLES

mysql> DESCRIBE player;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| pid | mediumint(8) unsigned | | PRI | NULL | auto_increment |
| name | varchar(40) | YES | | NULL | |
| model | varchar(30) | YES | | NULL | |
| rating | float | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> DESCRIBE game_player;
+-------------+-----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-----------------------+------+-----+---------+-------+
| gid | mediumint(8) unsigned | | | 0 | |
| pid | mediumint(8) unsigned | | | 0 | |
| ptimeactive | mediumint(8) unsigned | YES | | NULL | |
| pteam | varchar(30) | YES | | NULL | |
+-------------+-----------------------+------+-----+---------+-------+

mysql> DESCRIBE weapon_t;
+----------+-----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-----------------------+------+-----+---------+-------+
| gid | mediumint(8) unsigned | | | 0 | |
| pid | mediumint(8) unsigned | | | 0 | |
| wid | tinyint(3) unsigned | | | 0 | |
| kills | mediumint(8) unsigned | | | 0 | |
| deaths | mediumint(8) unsigned | | | 0 | |
| suicides | mediumint(8) unsigned | | | 0 | |
+----------+-----------------------+------+-----+---------+-------+

pid = player's id
gid = game's id
wid = weapon's id

I want to make a table that has the following.
player's name, player's id, total time active, total kills

The best SQL command I got is the following.

SELECT player.name, player.pid, SUM(weapon_t.kills) as kills, SUM(game_player.ptimeactive) AS timeactive FROM player LEFT JOIN game_player ON game_player.pid = player.pid LEFT JOIN weapon_t ON player.pid = weapon_t.pid GROUP BY player.pid


This command though gives me inaccurate results for kills and timeactive. Probably from a doubling effect or something.

yes, a cross-join effect

without subqueries (version 4.1) your best bet is to do something like this:
if you're not on 4.0 you will have to run the two queries separately