Athletic Question: What is some good foods to gain more calories/weight?

I do swimming almost every single day. (Swim Team Stuff).

I've been issues with keeping my weight up it keeps going down slightly.

For breakfast I eat eggs for lunch I usually have a travel pack of tuna, for dinner I have grilled chicken and broccoli.

I don't want to overdue food, I also need to stop my weight from going even farther down.

Is there any ideas or any meals that you guys eat that maybe I should try?

My weight is 45.8kg right now, sorry I don't know what that is in pounds.
Updates
1 mo
I have gained a whole 2 lb 🥳
Athletic Question: What is some good foods to gain more calories/weight?
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