Guys, he's using a Weber Spirit 3 burner gasser, not a kettle
Welcome Gillian,
Congrats on your new grill. The warming rack should get a bit less heat than the grate directly below it, but remember, heat rises. it doesn't just go from the burner to the grate then disappear. If you really need to just keep something warm for a period of time, turn off one of the burners and leave whatever you want to hold there. With 3 burners, you have some flexibility. You could shut down either the left or right burner and move the sausages over there, leaving the other 2/3'rds of your grilling area for direct heat. Or, you could turn off the middle burner and leave your sausages there, which will leave you with the left third and the right third for direct grilling.
If the hamburgers were hard, I'd wager that they were over cooked. Either they should have been taken off the grill earlier, or cooked at a lower temp setting on the burners.
On getting everything done at the same time, that will take some practice. You have to learn what heat levels to cook which food at, how long they will take, and how to shift gears in mid stream so to speak to have it all come out right. For example, grilled chicken, burgers and sausages. The grilled chicken will take the longest, so it should go on first. I like to sear my chicken so I'd fire up the grill with all 3 burners on high. Lay the chicken down starting on the left of the grill. Exactly where on the grate depends on how many pieces of chicken you are cooking. If only 2 or 3, then start at the back left directly over the burner tube with the first one, the second one in the middle and the third one at the front, making a line of chicken going back to front. Give the chicken a couple minutes on that side to sear and get some grill marks, then flip the chicken. Lid can be up or down, it's your choice. Lid down will actually speed things up, but lid up lets you be sure what is going on. Anyways, after the chicken has been seared on it's other side, turn that burner down to between medium and low. The chicken needs to cook more, but too a heat will burn it.
At this point, throw the sausages on over the middle burner and the burgers over the right burner. The sausages will need to be turned first as they will blister fairly quickly. Rotate the sausages, or flip them, or whatever to get a nice color on every part of them, then turn the middle burner down to low. Check the chicken and see how it's doing. IF it's done, shut the burner down but leave the chicken on the grate. If chicken isn't done yet, see if any of them need to be flipped. Also, you might notice that some of the chicken is cooking faster than the other pieces. If so, swap places. Grab the piece that is more "done" and move it to where a less "done" piece is, moving the less "done" piece to the hotter area.
While all that is going on, check on your burgers. Flip if needed. After both sides of the burgers have been seared, this is where judgment/experience comes in. What meats still need more time ? Can you crank up the heat under that meat ? Or does it need to cook at lower temp? If the chicken is done, and that burner is turned off, what about the sausages ? Are they ready and the burgers aren't ? Does the flame need to be turned down on the sausages or even off ? Who knows, maybe the burgers are done and the sausages aren't ? So move the burgers over next to the chicken, with the middle and left burners off, and move the sausages over above the right burner, which is on low. Close the lid and let the sausages finish.
All the above might sound daunting and complicated, and the first couple of times you try it, it probably will be. But, you'll get the hang of it and it will be old hat soon enough. When you break it down, it's really rather straight forward.