Please, I am findimg it difficult to solder on the collector part of the 2N3055. How can I solve this problem-because that is really disturbing me. Thanks and God bless you.
For all we know, he is referring to the terminal. He didn't say..
Is the part in a power package with lots of heatsinking? Try pre-heating the part with a hot air pencil and turn up the temp on your soldering iron (or get a higher power one)
The TO3 one has, fairly obviously! - it's a MUCH more substantial encapsulation.
It's perfectly possible to solder to the case of a TO3, and I've seen it done commercially - but it's really a pretty pointless occupation, and it's not the way it's designed to be used. Bolt it on a heatsink, with the correct mounting kit, and use a solder tag under one of the bolts - as it's supposed to be used.
Most TO-3's are sold with mounting bolts, nuts, washer(s), and a terminal lug that fits under one of the bolts. If the one you bought doesn't have the mounting kit you can buy them seperately.