It is difficult to answer the question without doing several time-consuming things. I will, however, tell you how to do them.
The question as to their worth depends on what happened during the mergers. Other firms purchased all three firms or were broken into different parts, but all three still exist in some form.
No one can compel you to sell your shares so if you did nothing during a merger and the firm did not partially liquidate or wholly liquidate as part of the merger, then you still own those shares.
In the case where the subsidiary firm still exists, and they did not cash out small shareholders, then you are a minority shareholder. You still are a fractional owner of those subsidiaries. You are entitled to any dividends if they are declared. If you have gone missing and they do not know even if you are still alive, the shares have probably escheated to the state you lived in at the time. The first thing you should do is contact the companies directly or the parent. You may also have dividends in possession of the state.
In some mergers, minor shareholders are paid out in cash because it costs too much to transfer small positions. They may have legally liquidated and sent you a check. You are entitled to that check.
The shares are likely escheated, but they may still be in your name. It is strange you have received no communications in all of this time. Part of the Motorola shares are part of a public firm, and you can sell them on the open market. The other firms are now private. There is no market for their shares. You are entitled to any dividends, and if the firm were liquidated, then you are entitled to any residue.
If they are in your name, you just need to update your information with them so that you can vote at the subsidiary shareholders meeting, which will be meaningless as you own a few shares and one shareholder holds millions. Nonetheless, if it passes income from subsidiary to parent, that is a dividend, and you should get a check.
If they have been escheated to the state, then you need to go through the state's process to reclaim the shares and any dividends. If you have never received a dividend check, you should still check with the unclaimed property division of the state you lived in at that time.
The buying firm may want to be rid of you. It is possible that they will make you an offer, but there is no requirement that they do so. They may also just ignore you. You own part of the company, but if there are no dividends and they are assured control, then they are obligated to manage the subsidiary for the highest return, but they are not required to actually issue a check to anyone, including the new parent.