monjursayad

But when I try to fight in arena they have many powerful legendary weapon like heron mark sword and more

Forget about getting legendary. If you do not get them focus making them as I did. It does require to grind a lot but can be done. Get rank stability and that grants you gold that you will know will get in a stable way. Max those parts in an even way. If you max them one at the time to reach max level will be hard to stabilize.  Get 10 levels at the time per modules at epic level then another 10 then 5 and another 5. When your set is max on L30 then focus into make legends from epics not in need but careful to meat possible good weapons. 
Focus in one mech at the time. Focus to fight single and dual not 3v3. I guess maybe you are in a clan doing war so that will be different. If is about pvp then focus 1v1 and 2v2 for now maxing those parts one mech at the time. You won't regret. 

You do not need any legendaries right now, except the ones you get from upgrading your epic modules.

Hi
First of all you need to indicate some of the basics things to understand your play ways
1- How much you can farm everyday? If your grinding it is very low the development will be really terrible. Trust me on that.
2- You have an advantage over me when I was under development and I had not decent modules or better say no epics modules to myth them. Regardless those are the old ones like the cooling, booster and energy and heat modules apart of the others, you need to grind really fast to max them up. 
3- Do not meat the epics parts given to max  out parts. Those are your legends future parts for transformation. 
4- Focus into max  Cooling booster, energy booster, heat engine and energy engine. That will give you rank stability and will survive countless fights with current mech power. 
5- Do not waste gold increasing power for now. Max to Myth level the modules. Now, careful to max too many since you can get a new modules and having too many of the old ones can become a waste but for current stability you need to go for it. 
The reason why they kill you fast is because you do not have cooling, heat, regen energy enough to survive. You have an average of 150 in cooling and  energy regeneration and lack of enough reserve. When you use your mech weapons those weapons also consume energy and heat forcing your mech to overheat and drain your energy regardless the physical attack against your mechs. Not just you overheat and lose energy using your own weapons rending you but your opponent will kill you in 2-3 shots. 
Get those fast to myth level and you will see the change. First those 4 below. Focus on this. 
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You can do some of this too but careful because those do not help in cooling or energy recovery as I can see in your set up. 

 

HI, My lvl on supermech is 159. But I don't have enough powerful weapon for my mechs. These 3 mechs are such weak mech. I have lack of legendary items. Give me some tips to develop these mechs. Here you can see my 3 mechs

Thanks for help
Thanks for help

Thanks for help
Thanks for help

Jam gave the answer to use OD6 insane. It is the best to use for fast grinding. If you have enough power then use that one. The cost of fuel 7 vs gold 9,600 and XP 19,000 is the best of course as fast. The amount of mechs and structures to destroy are minimal making auto process really good. Also, if you are trying to level up your account also it is the best on XP with gold granted and fuel usage. Few others options are harder but can give some fortune boxes but if is about gold-level up fast at low cost this is the holy grail. 😃

Overlord Den Mission 6 Insane give me 10000 Gold and take 7 fuel. Is it okay?

Thanks

But Crafting legendaries in base take too much time and coins

Thanks